1)  Zenyatta:  Zenyatta's Harmony Orb can be useful on Widowmaker if she needs a quick heal, but it's generally not worth it for Zenyatta to keep it on her.  Widowmaker is usually within range and line of sight of Zenyatta to receive the Harmony Orb whenever she needs a quick heal.

Zenyatta's Discord Orb can make shooting down enemies much faster for Widowmaker and it can increase the use of her non charged sniper shots, enabling Widowmaker to have more opportunities to land sniper shots on the enemy.  Since Zenyatta and Widowmaker stay somewhat close to each other, the target who has the Discord Orb is usually within Widowmaker's sight.

Widowmaker's Infra-Sight can allow Zenyatta to rev up charged volley orb shots against enemies just about to come around walls/corners and to make more informed decision on avoiding offensive flankers.

Zenyatta's ultimate Transcendence doesn't really pair too well with Widowmaker, since she tends to stay back away while sniping and usually can't take advantage of the healing benefits of Transcendence.  It can however provide some distraction for you while sniping, which could prevent enemies from firing at you during Transcendence.

2)  Symmetra:  Widowmaker benefits from Symmetra's Photon Shields as it allows her to withstand bursts of damage while sniping, and it can delay the need for her to grapple to health packs, keeping her in a sniping position more often.

Symmetra's Sentry Turrets can guard choke points leading up to Widowmaker's position, potentially protecting her from offensive flankers.  With this in mind, it's good to know where Symmetra puts her Sentry Turrets at so you know where it's safer to snipe from.  This can also let you be more informed where the vulnerabilities to your position are and you can put down your Venom Mine in any other vulnerable areas where the enemy can flank you from.

Even though Widowmaker can grapple back to the fight a tad faster and start firing from long range, she can still take advantage of Symmetra's ultimate Teleporter to get back to the fight even faster.

Infra-Sight is not overly powerful with Symmetra because her long range weapon (Photon Orbs) fires very slowly and can be hard to land shots on most enemies regardless if they are in view or range.  But Infra-Sight can help her make more informed decisions on which direction to spam her Photon Orbs at.  Infra-Sight can also warn Symmetra of incoming flankers, giving her enough time to take cover from them.

3)  Lucio:  Lucio's auras are somewhat useful for Widowmaker.  Speed aura can help her get away from flankers and escaping enemy ultimates.  Heal aura can keep Widowmaker healed up and let's her focus more on sniping rather than reaching for health packs occasionally.  Keep in mind that Lucio's auras only work when you stay within 30 meters of each other and keep line of sight with him as well.

Normally Lucio and Widowmaker are generally not close together, but if they are, Lucio can use his Soundwave ability to knock back deadly close up enemies from her such as Mei's freeze gun, Reinhardt's rocket hammer and Reaper's shotguns.  But Widowmaker can also use her Grappling Hook to get away from these same enemies as well.

Widowmaker's Infra-Sight can allow Lucio to see enemies before they come around a corner, giving him more precise control over spamming his sonic projectiles.  Infra-Sight can also help Lucio plan to knock an enemy into a pit with Soundwave as they walk around a corner, this tends be helpful on many Control maps.

Lucio's ultimate Sound Barrier is not too powerful with Widowmaker, but it can enable her to be more aggressive at grappling closer to the enemy and taking a few good shots without any harm.

4)  Mercy:  Although Widowmaker can use her grappling hook to reach health packs a bit quicker, she still benefits greatly from Mercy's heal.  Mercy's Guardian Angel ability enables her to jump around to different allies to heal them up if they need it.  The only issue with supporting Widowmaker as Mercy is they tend to be somewhat apart from each other most of the time.  Widowmaker can snipe at different perches and ledges, this can potentially expose Mercy to enemy fire more and it can take up to a few seconds to reach to her.

Mercy's damage boosting can be beneficial with Widowmaker's Sniper Rifle, especially right after healing her up since she is with her at the time.  But Mercy is better off not sticking around for too long, as she is needed to heal other allies most of the time anyways.

Infra-Sight is not overly powerful with Mercy.  Infra-Sight can warn Mercy of incoming offensive flankers, enabling her ahead of time to quickly fly to another ally to support instead, but that's about it.

Mercy's ultimate Resurrect only has mediocre use with Widowmaker.  After dying, Widowmaker can use her Grappling hook to quickly get back in the action and start firing away from long distances with her Sniper Rifle.  Also Resurrect usually only works on Widowmaker about half of the time since they tend to be further than the 15 meter distance that Resurrect works at.

5)  Ana:  Both Ana and Widowmaker are snipers, but they have completely different uses on the battlefield.  Widowmaker is pure damage output while Ana is mostly a support healer.

Ana can give Widowmaker Biotic Rifle heals when she needs it, which can keep her sniping shots going non-stop.  Widowmaker should communicate with Ana to let her know to heal you when you need it, otherwise you will have to grapple to a health pack which will disable you from combat for a short period of time.

Infra-Sight allows Ana to launch a Biotic Grenade or Sleep Dart at enemies coming around corners and can warn her of incoming flankers.

Widowmaker is low on the list of the best candidates for Ana to Nano Boost, mostly because as a backline sniper she does not take much advantage of the increased speed or damage reduction, but the increased damage output of Nano Boost can make Widowmaker very deadly.

1)  Winston:  Winston's Barrier Projector can be very beneficial for Widowmaker.  Just like with Reinhardt's Barrier Field (shield), she can safely dish out damage from inside Winston's Barrier Projector.  Although Widowmaker tends to snipe from random perches and ledges and a lot of the time she is not near Winston to take advantage of his barriers.  But if Winston is on your team, consider sticking around him a bit more if you can to take advantage of his barriers occasionally.  If Widowmaker is in trouble somewhere, Winston can Leap to your rescue and then put down a barrier to further protect you from flankers.

Widowmaker's Infra-Sight can allow Winston to start leaping towards an enemy right before they come around a corner, potentially taking the victim by surprise and allowing Winston to kill the victim with his Tesla Cannon.  Infra-Sight can also help Winston's ultimate Primal Rage by giving him more information on where to chase and leap after his enemies more effectively.

Winston's Primal Rage can provide a lot of distractions, potentially allowing Widowmaker snipe away without getting targeted as much.

2)  Zarya:  Zarya's Projected Particle Barriers can help Widowmaker survive in harsh situations, especially when she is getting flanked.  But since Widowmaker is a defensive hero she is usually not a prime candidate for Zarya's Projected Barriers, like offensive allies usually are.

Widowmaker's Infra-Sight can allow Zarya to lob her explosive charge shots more effectively before enemies come around walls/corners, and it can allow her to make more informed decisions on where to lead her team to.

Zarya's ultimate Graviton Surge doesn't really have much synergistic value with Widowmaker.  If you can, lob a Venom Mine at the clumped up enemies as it will deal a damage over time effect to all enemies stuck in the surge.  Otherwise, sniper shots is the only other solution.

3)  Reinhardt:  Reinhardt's Barrier Field (shield) can be valuable to Widowmaker.  Just like with Winston's Barrier Projector, Widowmaker can safely stay out in the open and dish out plenty of DPS behind his shield.  Although Widowmaker tends to snipe from random perches and ledges and a lot of the time she is not near Reinhardt to take advantage of his shield.  But if Reinhardt is on your team, consider sticking around him a bit more if you can to take advantage of his shield occasionally.

Widowmaker's Infra-Sight can allow Reinhardt to launch a Fire Strike before an enemy comes around a wall/corner, potentially getting an easy hit on an enemy.  Infra-Sight can also allow Reinhardt to start doing a Charge even before an enemy comes within view.

Reinhardt's ultimate Earthshatter can give Widowmaker easy sniping shots at the dropped/stunned enemies.

4)  D.va:  D.va can use her Boosters to knock down enemies making it easier for Widowmaker to fire sniper shots at the knocked down enemies.  D.va's Defense Matrix can provide some nice temporary protection for Widowmaker allowing her to safely stay out in the open to dish out sniper damage.

If Widowmaker is getting flanked, D.va can boost to her rescue and then turn on Defense Matrix for protection.

Widowmaker's Infra-Sight can give information to D.va allowing her to use her Boosters more effectively to flank enemies and to stay out of too much danger.  It can also allow D.va to launch her ultimate Self-Destruct towards a bunch of targets hidden behind walls.

5)  Roadhog:  Widowmaker's Infra-Sight allows Roadhog to prepare a Chain Hook before an enemy comes around a corner/wall.

Roadhog's ultimate Whole Hog can scatter enemies around giving Widowmaker an easier time to pick off disoriented enemies from the knock backs of Whole Hog.

1)  Junkrat:  Infra-Sight is extremely beneficial for Junkrat.  Infra-Sight enables Junkrat to bounce his grenades into rooms and hallways to hit enemies that he can see behind walls.  This is a very efficient synergy between these two heroes.

Infra-Sight can also help Junkrat steer his Rip-Tire more effectively, since he will be able to see how many enemies are in which rooms, etc.

Both Widowmaker and Junkrat have traps they can put down to deal with offensive flankers and ambushes.  Junkrat has Traps/Mines that can be placed in similar fashion to Widowmaker's Venom Mines to provide an even stronger defense to guard choke points or from enemies creeping up on your team.

2)  Torbjorn:  Infra-Sight doesn't help Torbjorn as much as other heroes, since Torbjorn's turret cannot take any advantage of it but Torbjorn's Rivet Gun can be aimed slightly ahead of time to shoot at enemies coming around corners.  Infra-Sight can also make it safer for Torbjorn to run around and pick up Scrap, as he will be able to see any threats approaching his way.

Venom Mines can be placed in a position to protect Torbjorn and his turret from offensive flankers.

Widowmaker can always grapple towards Torbjorn's turret if she needs a place to provide protection from flankers.  This can also lure enemies into the turret and Torbjorn can activate Molten Core if she brings a lot of enemies with her.

Widowmaker greatly benefits from Torbjorn's Armor Packs since she is a relatively squishy hero and can normally be headshotted to death from full health with one hit from some weapons in the game.  Armor increases her survivability against offensive flankers, allowing her to grapple away from them in time safely.

3)  Soldier 76:  Infra-Sight allows Soldier 76 to shoot his Helix Rockets right before an enemy comes around a corner.  This also makes using Tactical Visor (his ultimate) safer and more effective.  Soldier 76 can also chase fleeing enemies more effectively with Infra-Sight as well.

Most of the time Widowmaker is out on a perch sniping enemies and is usually too far away to take advantage of Soldier 76's Biotic Fields.  Besides, Widowmaker can usually grapple to health packs faster if she needs healed.  If your not near a health pack, sticking near Soldier 76 for some healing is a good idea.

Widowmaker excels at killing all of Soldier 76's counters, including:  Bastion, Reinhardt, and Zarya.

4)  Mei:  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 Mei more informed decisions whether its safe or not to go in the front lines to freeze enemies with her freeze gun.

Freezing enemies (with either Blizzard or the her freeze gun) 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.

5)  Pharah:  Infra-Sight can let Pharah see enemies before they come around a corner, enabling her to start firing rockets before they come within her line of sight, which happens to be very effective for Pharah.   Infra-Sight also allows Pharah to scout the area out to see if it's safe to use a Jump Jet.

If Widowmaker is getting flanked, Pharah can use a Concussive Blast on the flanker to knock the enemy away from her.

Widowmaker is strong against many of Pharah's counters, including:  McCree, Hanzo, Zenyatta, and Bastion.

1)  Bastion:  One of Bastion's drawbacks is he cannot shoot at very long ranges, but Widowmaker covers long distances with her sniper rifle very well.

Infra-Sight doesn't help Bastion as nearly as much as other allies, but it can help him see flankers coming up on him better along with allowing him to being able to get ready to shoot at enemies before they come around a corner.  Infra-Sight also pairs well with Bastion's ultimate Tank Form, as it can allow him to shoot at enemies just around corners, as the splash damage can still reach to them.

Widowmaker can place her Venom Mines in a spot that protects a choke point leading up to a Bastion Sentry.  This can provide some protection from flankers.

2)  Hanzo:  Both Hanzo and Widowmaker have similar roles, abilities and uses on the battlefield.  They are both snipers that are good at dealing with enemies from long range.  They both have ways to maneuver around a map (Widowmaker's grappling hook and Hanzo's climb ability).  They both have ways to see enemies behind walls (Widowmaker's Infra-Sight and Hanzo's Sonic Arrows).  They both have a utility weapon to deal some extra damage to enemies (Widowmaker's Venom Mines and Hanzo's Scatter Arrow.  However, where things are a bit different is Hanzo's ultimate Dragonstrike, as there is nothing else in the game that is really similar to it.

Even though Hanzo's Sonic Arrow works much like Widowmaker's ultimate Infra-Sight, Infra-Sight is simply much more powerful since it lasts longer and lets you see all the enemies on the entire map.  With Hanzo, this can allow him launch a Dragonstrike much more effectively and it can also allow him to position himself on the map better to prepare to deal with his enemies better.

Even Hanzo's Sonic Arrow is still useful for Widowmaker.  When Infra-Sight is not available, Sonic Arrow can allow her to constantly be ready to fire her Sniper Rifle at enemies coming around corners and to make more informed decisions on where to grapple to.

Widowmaker can place her Venom Mines in places where it can potentially protect Hanzo from offensive flankers, providing a little extra DPS to finish off any wondering enemies for him.

3)  Reaper:  Infra-Sight lets Reaper know if using Shadow Step at a certain destination is safe or not, this can also help plan Reaper's Shadow Wraith paths as well.  And overall Infra-Sight is very useful for Reaper to plan flanks and keeping out of too much danger in general.

Reaper can also harass long range damage dealers that Widowmaker can have trouble with, including:  Soldier 76, Torbjorn, McCree and Hanzo.  Keeping these enemies busy dealing with Reaper will let Widowmaker freely do her dirty work.

4)  McCree:  Infra-Sight can help McCree prepare for using his Flashbangs, as he will be able to see enemies right before they come around a corner.  However, due to the instant hitscan nature of McCree's revolver, Infra-Sight is not overly powerful with McCree.

Widowmaker can place her Venom Mines in places where it can potentially protect McCree from offensive flankers, providing a little extra DPS for McCree to finish off any wondering enemies.

There is a few enemies that McCree is weak against that Widowmaker is strong against, and that includes:  Bastion, Hanzo, and Zarya.

5)  Genji:  Genji's distractions can enable Widowmaker to do her dirty work from afar without being targeted as much.

Infra-Sight helps Genji to track down fleeing enemies, allowing him to get a Swift Strike in to finish a victim off.  Infra-Sight can also enable him to start firing shurikens before an enemy comes around a corner.

6)  Tracer:  Tracer's distractions allow Widowmaker to do her sniping responsibilities from afar without being targeted as much.

Infra-Sight can help Tracer make better decisions on where to Blink to, or which room or hallway to go into or avoid.  Tracer can plan Pulse Bomb throws easier with Infra-Sight as well.

7)  Widowmaker:  Two Widowmakers is usually a poor choice because the enemy will most likely end up going mostly with offensive flankers to counter the widows.  Besides providing twice the sniping power there really isn't any synergies two Widowmakers can do together.

Multiple Venom Mines do not stack.  If an enemy gets struck by a second Venom Mine it will have no effect at all and it will simply get "wasted".

The biggest tip I can give you is to spread out your abilities with the other Widowmaker.  This means use Infra-Sight at a different time than the other Widowmaker as two Infra-Sights do not do anything extra together and you'll just be wasting your ultimate.  Spread out your Venom Mines as well.  I also recommend sniping at a different area than the other Widowmaker as you will be protected more from flankers and gun shots in general.