1)  Zenyatta:  Zenyatta's Harmony Orb is not overly useful with Mei since she can just use Cryo-Freeze to heal herself, but like with other healers she can take advantage of the extra healing and remain in the front lines longer to focus on freezing targets without worrying about using Cryo-Freeze as much.  Zenyatta is usually better off putting his Harmony Orb on a different ally, but if Mei is trying to freeze enemies up close she becomes a prime target for it.  Just like how a Cryo-Freeze will instantly remove an enemy Zenyatta's Discord Orb, it will also remove the Harmony Orb, but at 3/4 of the Cryo-Freeze duration.

Frozen targets are actually easy for Zenyatta to attack at with his energy orbs, which can allow Mei to continue freezing more targets instead of using Icicles to finish off the frozen victims.

Enemies who have received the Discord Orb are great targets for Icicle shots, so be mindful on which enemy Zenyatta puts his Discord Orb on and focus on killing that enemy before others.

Sometimes it's beneficial to prevent an enemy from losing sight with Zenyatta who has received the Discord Orb.  Mei can achieve this by using her Ice Wall to block an enemy in to prevent this from happening.  Ice Walls can also be used to block enemies away from Zenyatta to protect him from offensive flankers.

Zenyatta's ultimate Transcendence can allow Mei to be very aggressive at moving in the front lines to freeze enemies with her freeze gun and preventing enemies from running away with Ice Walls.  But besides that there really isn't any other synergies with Transcendence because Mei is already self sufficient with her Cryo-Freeze to heal up damage.

2)  Mercy:  Mei is actually one of Mercy's least useful allies to support.  Mei's Cryo-Freeze is what makes her not the best target for allies to heal, and this holds true for Mercy as well.  Mei is also self sufficient thanks to her Ice Wall ability to block shots from the enemy from hitting her and also allowing her to escape from deadly situations.  Mei benefits the most from Mercy's healing beam when she is up close trying to freeze enemies, as it will enable her to keep the frost stream going without a delay from Cryo-Freeze.  Also keep in mind that Mercy generates ultimate charge when healing her allies, so sometimes you should just let her heal you instead of using a Cryo-Freeze in certain situations.

Unfortunately, Mercy's damage boost does not allow you to freeze enemies any faster with the freeze blaster.

You can use an Ice Wall to protect Mercy while you focus on freezing targets up close.  Just place your Ice Wall so Mercy can still heal you while you focus on freezing targets.

Mercy's damage boosting is only mediocre at best with Mei.  Damage boosting Mei's Freeze gun does increase the DPS but doesn't affect the freeze times. Damage boosting her long range Icicle shots can be beneficial but are not as important as other weapons in the game.  Damage boost increases Blizzard's damage, but it does not freeze enemies any faster.

Mei is considered low on the list of allies that are safe to follow.  A lot of this has to do with Cryo-Freeze.  When Mercy is following Mei around, a Cryo-Freeze can just leave Mercy hopeless without no one else to fly to sometimes.

Due to Mei's poor mobility to get back to the fight faster after dying (besides her Ice Wall boost) she greatly benefits from Mercy's Resurrect.  Mei's defensive nature usually keeps her within Mercy's range for Resurrect to work on her.

3)  Symmetra:  Symmetra's Photon Shields are helpful for all heroes, but Mei can heal herself with Cryo-Freeze which makes having Photon Shield less important than other heroes, but Mei can still benefit from it by allowing her to stay close up with her enemies longer and more safely to allow her to freeze enemies with her freeze blaster.

Freezing enemies up close with either Mei's freeze blaster or Blizzard can give Symmetra easy targets to use her Projector Beam on to help rev up her ultimate.  Symmetra's Photon Orbs travel very slow, so it generally doesn't pair so well with frozen targets.

Symmetra's Sentry turrets slow enemies just like Mei's freeze blaster.  The two combined can make it incredibly hard for an enemy to even move or escape once caught in this combo.  With this in mind, it can be beneficial for Symmetra to spread out her Sentry Turrets so it gives Mei an easy opportunity to "trap" wondering heroes around the battlefield with her freeze gun and Ice Wall.

Ice Walls can be used to protect Symmetra from offensive flankers by blocking them away from her.  Please note that Symmetra is unable to put her Sentry Turrets on Ice Walls.

Mei has extremely poor mobility (besides using Ice Walls to propel herself to higher areas) so she greatly benefits from Symmetra's Teleporter to get back to the fight faster.

Mei's Blizzard can make it safer for Symmetra to go up close to use her Projector Beam to easily take out the chilled/frozen victims.

4)  Lucio:  Mei benefits a lot from Lucio's auras, especially his speed aura.  Speed aura allows Mei to be more aggressive with her freeze gun and to move in and out of combat seamlessly.  Heal aura is only somewhat useful with Mei as she has Cryo-Freeze which she can use to heal herself anyways, but with a constant healing it can allow her to continue to freeze enemies up close without worrying about using Cryo-Freeze as much.  Keep in mind one must keep line of sight with Lucio and be at least 30 meters in range of Lucio in order to receive his auras.  Also when Cryo-Freeze is used, Mei is unable to receive Lucio's auras or his ultimate Sound Barrier.

Lucio's Sonic Projectiles travel somewhat slow, so it's not the best weapon to use against frozen targets.

Lucio's knockback ability (Soundwave) is not too important around Mei since she can use Ice Walls to block deadly close range attackers away from her, and she excels in close range combat herself with her freeze gun and Cryo-Freeze.

Lucio's ultimate Sound Barrier can allow Mei to be aggressive up close with her freeze gun for a short period of time, but unfortunately Blizzard does not have much synergistic value with Lucio, and Lucio is usually better off saving his Sound Barrier at a different time Mei uses a Blizzard.  The extra shields from Sound Barrier are no use against frozen targets from a Blizzard.

5)  Ana:  Mei can be very effective in the front lines to freeze enemies with her freeze blaster.  Ana can keep firing Biotic Rifle healing shots at Mei to keep her in the front lines freezing enemies non stop.  This can be especially potent when combined with Ana's Biotic Grenade.  This can also delay Mei's Cryo-Freeze for a later time.

Frozen enemies are easy targets for Ana's Sleep Darts, but its usually a better idea to just kill off the frozen enemy instead with quick headshots.

Unfortunately, Mei's Cryo-Freeze is the only healing ability in the game that Biotic Grenades have no synergistic use with.

Since Ana has no vertical mobility abilities, she can take advantage of Mei's Ice Wall boost to reach to higher places she can normally not get to.  When higher up, Ana can be safer from certain enemies such as Reinhardt and flankers like Tracer.

Nano Boosting Mei (Ana's ultimate) is only a mediocre choice.  Mei's short range frost stream is a tad on the weaker DPS side and her purpose is to freeze enemies and then let her allies take the frozen enemies out.  Even though Mei's frost stream will do a bit more damage with Nano Boost, it won't freeze enemies any faster, so her frost stream does not benefit a whole lot from Nano Boost.  However, the increased speed and damage reduction allows her to move up close to her enemies quicker to make her a harder target to take down, allowing her to possibly freeze more enemies.  Nano Boost also does not effect freeze times of Blizzard, but it will do more initial damage from Blizzard.

1)  Reinhardt:  Reinhardt's shield can allow Mei to safely get behind and freeze enemies up close or shoot Icicles from afar without worrying about using Cryo-Freeze to save her as much.

In close range both Mei and Reinhardt can make a devastating team.  Reinhardt's Rocket Hammer can knock down multiple enemies while Mei freezes them.  This combo can make it incredibly hard to escape out of.  On top of this Mei can use Ice Walls to prevent enemies from escaping as well.

Reinhardt's ultimate Earthshatter is another great way to drop/stun enemies, preventing them from escaping Mei's frost stream or Blizzard.

2)  Zarya:  Zarya's Projected Barrier can be useful on Mei when freezing targets up close with the freeze gun.  This can keep Mei going strong without having to use Cryo-Freeze and can potentially keep Zarya's weapon charged up.

Zarya's slow mobility can make it hard for her to run from the enemy when overwhelmed.  Mei can use Ice Walls to block them away from Zarya to allow her to recuperate faster.

Zarya's ultimate Graviton Surge doesn't really have any synergies with Mei.  If you see a Graviton Surge the best thing to do really is shoot Icicles at the clumped up enemies.  It's generally too risky to go up close with the attempt to freeze the clumped up enemies as they are still able to shoot and use their abilities.  If your allies cannot provide enough fire power to kill the enemies stuck in the surge then a Blizzard can be a decent backup plan, as the enemies will be frozen by the time the Graviton Surge ends.

3)  D.va:  D.va can use her Boosters to knock down enemies, which can give Mei more time to safely freeze enemies up close.  Dva's Boosters also pair well with Blizzard.  Throw down a Blizzard, then let D.va Boost through all the enemies over the Blizzard to knock them down, preventing them from escaping it.

Up close and D.va can protect Mei with her Defense Matrix, which will enable Mei to continue freezing enemies without worrying about using Cryo-Freeze as much.  Mei can also protect D.va by blocking shots with Ice Walls, this will enable D.va to dish out plenty of damage up close with her Fusion Cannons.

When D.va uses her ultimate Self-Destruct, you can throw up an Ice Wall to prevent enemies from escaping the explosion.  Blocking hallways or rooms off with an Ice Wall can prevent them from losing line of sight from the Self-Destruct.  Blizzard can also pair well with Self-Destruct.  The Blizzard will slow/chill enemies enough to prevent them from running away from the explosion.  This is really only recommended if there are a lot of enemies to take care of.

4)  Winston:  Winston's Barrier Projector allows Mei to safely dish out damage with Icicles and throw down Ice Walls all while being shielded.  Barrier Projectors are also useful in close range, as it can allow Mei to freeze enemies up close while being shielded from everything else.

Winston is prone to damage from long range, so shielding him with Ice Walls can be effective if his barrier is on a cooldown.

When Winston uses his ultimate Primal Rage, it's generally a good idea to wait and use Blizzard at a different time, since Primal Rage will most likely knock enemies away from the Blizzard.  What Mei can do while Winston is using Primal Rage is prevent enemies from running away from it by blocking them off with an Ice Wall.

5)  Roadhog:  Roadhog is an independent tank that doesn't have many synergies with Mei.  Sometimes being up close trying to freeze enemies can be risky for Mei, but with Roadhog, he can Chain Hook one of the enemies so Mei has less enemies to deal with up close.

Keep in mind that Roadhog can also Chain Hook frozen enemies into him.  Frozen stationary enemies are easy Chain Hook targets for Roadhog and it can be useful for him to Chain Hook frozen enemies into him if he gets the chance.

Mei can assist Roadhog with Ice Walls to block enemies from going after him while he uses his Take a Breather self-heal ability or even with enemies chasing after him due to his slow mobility.

Roadhog's ultimate Whole Hog doesn't have much synergistic value with Mei, and Mei's Blizzard doesn't have much synergistic use with Roadhog as well.

1)  Widowmaker:  Widowmaker's ultimate Infra-Sight allows Mei to see enemies before they come through doors or choke points, this can allow her to throw up an Ice Wall in time to block them out.  Infra-Sight can give you more informed decisions whether its safe or not to go in the front lines to freeze enemies with the freeze gun.

Freezing enemies can give Widowmaker easy shots with her sniper rifle, which can allow Mei to continue freezing more enemies instead of using Icicle headshots to kill the frozen victims.

2)  Torbjorn:  Torbjorn can provide armor packs for Mei, which will provide some extra protection.  Armor obtained from armor packs cannot be healed and you will have to pick up a new armor pack to obtain more armor.  Luckily, Torbjorn is usually not to far from Mei and can get new armor packs easier than offensive heroes can.  Armor packs can enable Mei to be more aggressive with freezing enemies up close and delaying Cryo-Freeze a bit later.

Mei can prevent enemies from running away from Torbjorn's turret by walling them in with Ice Walls, or freezing them with either the freeze gun or Blizzard.  Ice Walls can be positioned so it can make it safe to run in the front lines to pick up Scrap.

Torbjorn's ultimate Molten Core can pair well with Blizzard, as the extra DPS from Molten Core can make easy work of the frozen victims.

3)  Bastion:  Mei can provide some protection for a Bastion Sentry through multiple means.  Mei can put an Ice Wall right underneath a Bastion Sentry to lift him up, leaving offensive flankers down in the dust.  This tactic is especially deadly to enemies like Reinhardt and his deadly Rocket Hammer, an enemy Mei and her freeze blaster and other enemies like Reaper and McCree.

Mei can also freeze enemies up close with her freeze gun to keep those same enemies away from a Bastion Sentry.  Frozen enemies are also very easy to shoot at for Bastion.

Mei has a powerful strategy with Bastion.  Mei can lift Bastion up with her Ice Wall to places where Bastion normally can't get to.  Bastion can then setup a Sentry in one of these high places.  This can provide further protection from offensive flankers and also helps a lot to stay away from deadly Reinhardts.  Since Reinhardt's abilities are mostly ground based, he has trouble dealing with enemies that are elevated high up.

Blizzard pairs very well with Bastion's ultimate Tank Form.  Blizzard will slow and then freeze enemies, giving Tank Form an easy time to shoot cannons at the frozen enemies.  Mei's Blizzard also works with a basic Bastion Sentry, allowing him to shoot frozen targets easily and quickly.

4)  Soldier 76:  Even though Soldier 76's Biotic Fields can be very helpful for Mei, she has her own self heal and is not dependant on other allies to heal her as much.  Biotic Fields can enable Mei to continue freezing enemies up close without being halted by using Cryo-Freeze.

Ice Wall can provide great crowd control for all her allies by blocking most of the the enemies off and only exposing a few enemies at a time, which provides great protection for Soldier 76.

Blizzard pairs very well with Soldier 76's Tactical Visor.  Blizzard will slow and then freeze enemies solid protecting Soldier 76 while using Tactical Visor and preventing enemies from running away from it.  Mei can also freeze single targets up close with her freeze gun, these can be easy targets for Helix rockets or rifle shots to the head for Soldier 76.

5)  Junkrat:  Mei's Ice Wall provides a nice cover for Junkrat, and Junkrat can still dish out damage by launching his grenades or mines over Ice Walls.  Even Rip-Tires can climb Mei's Ice Walls and then detonate, which can be a very powerful strategy for keeping tires alive as they approach their destination.

Another synergy with Ice Wall is Junkrat can put down his Trap/Mine combo right in front of the wall.  When the wall collapses the enemies will most likely try to run forward potentially getting trapped by the Trap/Mine combo.  The big ice chunks from the collapsed wall could camouflage the traps.

Mei's Cryo-Freeze can provide a nice shield for Junkrat to stay behind to take cover from while still allowing him to spam grenades.  Mei's Cryo-Freeze can also provide a shield to protect Junkrat's ultimate Rip-Tire as it reaches closer to the enemy.

Blizzard will slow and then freeze enemies, which can be easy targets for Junkrat to fire grenades or Concussive Mines at.  Even Rip-Tire pairs well with the chilled/frozen targets from Blizzard, preventing it from getting shot down.

6)  Reaper:  Reaper and Mei can be a devastating team close up to the enemy.  Mei can freeze targets with her freeze gun and Reaper can follow through with shotgun headshots to finish off the frozen victims easily.  While this is going on Mei can put up Ice Walls so they only have to fight no more than a few enemies at once.  If Mei needs healed she can just use Cryo-Freeze and Reaper can collect Soul Globes or Shadow Wraith to a health pack.

Blizzard works well with Reaper's ultimate Deathblossom.  Blizzard will slow and then freeze targets, giving Reaper a safe opportunity to execute a Deathblossom.

7)  Hanzo:  Mei can freeze targets up close and Hanzo can finish off the frozen targets with Storm Bow headshots, this can allow Mei to continue to freeze more enemies without worrying about using Icicles to finish off the frozen targets.

Hanzo can use Mei's Cryo-Freeze as a shield to get behind to help reduce damage.  Mei's Ice Wall can provide another shield for Hanzo to shield damage from.  Hanzo can also climb Mei's Ice Wall, allowing him to position himself differently to deal with ground heroes more effectively such as Reinhardt and an enemy Mei's freeze gun.

Hanzo's Sonic Arrow can provide a lot of information to Mei about when to put up an Ice Wall to block incoming attackers and prepares her to shoot Icicles before enemies come within view.  At the start of a match, Hanzo and Mei can figure out which door the attackers are about to come out at with the help of Sonic Arrows.  Mei can then get ready to throw up an Ice Wall once the match starts to lock them into the starting area.

Hanzo's ultimate Dragonstrike can pair well with Blizzard.  Blizzard will slow and then freeze targets, potentially making them unable to run away from a Dragonstrike in time.

1)  Genji:  Mei can freeze targets with her freeze gun, giving Genji an easy opportunity to finish off those victims with shuriken headshots.

Genji can climb Mei's Ice Walls, which can give Genji more possibilities for mobility to get around the map.  Genji can also climb to the top of an Ice Wall and attack enemies from unexpected angles.

Blizzard is a great opportunity for Genji to use his ultimate Dragonblade to finish off all the "chilled/frozen" targets.

2)  Tracer:  Mei can use her Ice Wall to provide crowd control for Tracer by blocking part of the enemy off from her, which can make harassing easier and more effective for Tracer.

Up close and Mei can freeze targets with her freeze blaster, which can allow Tracer to have easy headshots to finish them off.  Mei's Blizzard can chill and freeze targets as well, which can allow Tracer to get some easy Pulse Pistol shots in.

Tracer can use Mei's Cryo-Freeze as a wall to take cover from while harassing up close.

3)  McCree:  Mei can freeze targets up close and McCree can finish off the frozen targets with revolver headshots, this can allow Mei to continue to freeze more enemies without worrying about using Icicles to finish off the frozen targets.

McCree can use Mei's Cryo-Freeze as a shield to get behind to help reduce damage.  Mei's Ice Wall can provide another shield for McCree to shield damage from.

Blizzard is not overly powerful with McCree's ultimate Deadeye, but it can be an effective synergy.  Blizzard will slow enemies, but they can still fire like normal until they get frozen.  Once the enemies do get frozen they can all be easily killed anyways with regular headshots.  Unless there are a lot of enemies getting frozen, McCree is usually better off saving Deadeye for a different time.

4)  Pharah:  Frozen enemies from either the freeze gun or Blizzard is easy targets for Pharah's Rocket Launcher.

Blizzard can freeze a lot of targets which can give Pharah a safe opportunity to use Barrage.

Mei can use her Ice Walls for crowd control against long range attackers to hold them back long enough for Pharah to do some safe Jump Jetting in the air.

5)  Mei:  Two Meis can provide twice the amount of Ice Walls to block enemies off and create even bigger walls when two walls are put side by side.  But keep in mind that Ice Walls cannot stack on top of each other from multiple Meis.

Two Meis can keep a frost stream going non-stop if one has to use a Cryo-Freeze to heal up the other Mei can continue the freeze effect until the victim is frozen, this can ensure the success of freezing enemies more effectively.  But keep in mind that when two Meis are trying to freeze the same target, the chill effects do NOT stack and it will take the same (normal) amount of time to freeze enemies.  With this in mind it is better to attack a different target than what the other Mei is attacking with the freeze blaster.

There is no sense in putting two Blizzards on top of each other as the effects won't stack, so either wait until the other Blizzard is done to put yours up or spread them out to cover a larger area.