Bastion:  The reason why Junkrat is a good counter to Bastion is because of his Frag Launcher weapon.  The grenades from Junkrat's Frag Launcher can travel decent distances and bounce around walls or corners eventually ricocheting to a Bastion Sentry while Junkrat can often fire them out of Bastion's line of sight.

Junkrat's ultimate Rip-Tire happens to be somewhat effective against Bastions.  Tires can climb walls and possibly flank a Bastion Sentry from behind.  But remember a Bastion Sentry can shoot down a Rip-Tire very fast, so you need to be as sneaky as you can with it.

Although Bastion doesn't usually need to worry about Junkrat's Mines and Traps since he moves slow approaching them and is in Sentry Mode most of the time anyway.

Mei:  Junkrat has ways to counter all of Mei's abilities.  Junkrat can easily fire his grenades over Mei's Ice Wall dealing lots of damage safely.  Rip-Tires can also climb over her Ice Walls and kill enemies behind it.

If Mei uses a Cryo-Freeze (Mei Freezes in place, becomes invulnerable, and heals up) Junkrat can then put a Steel Trap down next to her, and she will be trapped after she unfreezes.  You can also take this a step further by placing a mine under the Cryo-Freeze as well, dealing even more damage.

If she gets too close to Junkrat and tries to freeze him, Junkrat can just use a Concussion Mine jump to escape it.  Same thing with Mei's ultimate Blizzard, Junkrat can simply do a Concussion Mine jump to get out of it.

Roadhog:  Junkrat is usually in the clear of Road Hog's abilities, especially his Chain Hook.  Since Junkrat is mostly in the distance lobbing grenades, usually behind a wall or object, he is safe from Roadhog's Chain Hook and Whole Hog ultimate most of the time.  Roadhog's poor mobility makes it harder for him to get close to Junkrat.  While Junkrat has far superior mobility with his mine jumps.

As Junkrat, keep your distance from Roadhog and you will be fine.  If you find yourself getting to close to him, you can use a Concussion Mine jump to get away from him.  Launching Rip-Tires from high above works great on Roadhogs.  If he actually does Chain-Hook you in, quickly backup while throwing down a Trap/Mine combo and then try to escape while lobbing grenades.

Junkrat's Rip-Tire has the capability to kill Roadhog at full health, and since Roadhog is such a slow moving hero, Rip-Tires can easily take him out.  Keep in mind that Roadhog can actually Chain Hook a Rip-Tire into him.  However, while the tire is being chained in, Junkrat can still detonate it at any time he wants to.

Tracer:  Fast moving ground targets are an easy prey for Junkrat's Steel Traps because it's very hard to spot them before you hit them.  Tracer's fast movement makes it difficult to dodge Junkrat's traps and this is the main reason why Junkrat is stronger than Tracer overall.  Although, Tracer can actually Blink past a Trap unharmed.

Dodging Junkrat's Frag Launcher can be quite difficult due to the randomness of the grenades bouncing around, these hindrances can make it difficult to get close to Junkrat as Tracer.

Tracer can avoid Junkrat's Trap/Mine combo by using Recall before he triggers his mine, potentially avoiding the damage from Junkrat's Mine.  Tracer is also decent at avoiding Junkrat's Rip-Tire by blinking away from it.

Junkrat can also use his mine jumps to escape away from Tracer as she has very little vertical mobility.

If Tracer manages to stick a Pulse Bomb on Junkrat, there is nothing he can do to counter it and will die unless he receives a Projected Barrier from Zarya or a Sound Barrier from Lucio.

Reinhardt:  Not many heroes have ways to deal with Reinhardt's massive shield, but Junkrat can shoot his grenades over his shield, potentially ricocheting off of a wall behind Reinhardt to deal a good amount of damage to him and his team.  Keep in mind that Reinhardt's shield will block everything Junkrat has, including:  mines, traps and frag grenades.

Thanks to Junkrat's mine jumps, he can get to higher ground and avoid most of Reinhardt's abilities, including:  melee attacks, Charge, and his ultimate Earthshatter.  Junkrat can also put a trap/mine down to stop a Charging Reinhardt coming after him.

Since Junkrat is usually able to take cover while lobbing grenades, he is virtually safe from Reinhardt's Fire Strikes.

Reinhardt should be a prime target for Rip-Tire as it can take him out at full health and there is not much he can do to avoid it other than attempt to throw a Fire Strike at it.  Remember shields will block Rip-Tire damage, but tires can also be steered through shields and then be triggered.

Torbjorn:  Torbjorn's turrets are everything to him, luckily Junkrat has the ability to fire at his turrets while taking cover.  Grenades can fly over to the turret and explode dealing massive damage to his turret and possibly Torbjorn himself.  Torbjorn's Turret does fire at all of Junkrat's abilities, including:  Steel Traps, Concussive Mines, and his Rip-Tires.

On the bright side for Torbjorn, he usually never has to worry too much about getting trapped and mined since he is usually in the distance building turrets and his slow movement quickly warns him if he is about to run over them.

Symmetra:  Junkrat has only a slight advantage over Symmetra.  Symmetra rarely comes into close contact with any hero in Overwatch since she is busy putting up turrets, teleporters, or lobbing Photon Orbs.  So she rarely needs to worry about running into one of Junkrat's traps, even though it can happen.  Still the traps will further help keep Symmetra's short range Photon Projector beam away from Junkrat.

Junkrat's Frag Launcher spam works well against Symmetra's Sentry Turrets.  If you know the enemy has a Symmetra on their team, fire random grenades in rooms or hallways before walking in there to destroy any potential Sentry Turrets.  Grenade spam can also work good on Symmetra herself as she is usually in the targetable range for it when she is firing her photon orbs.

It's not usually Junkrat's job of hunting down Symmetra's teleporter, but he does have decent mobility and can take some time out to go hunt one down if he needs to.

D.va:  Since D.va and her mech are such a big target, grenade spam works pretty well on her.  As long as D.va remains in the distance from Junkrat, he will do fine with grenade spam.  However if D.va comes in close range she will be able to do massive damage with her Fusion Cannons, in which you will need to escape it by throwing a trap at her and then mine jumping away from her (or at least one of them).

Be aware when D.va uses Defense Matrix, as it will absorb your Concussive Mines and frag grenades (but not your Traps).

D.va's Self Destruct ultimate can be easily avoided by mine jumping away from it, or simply hiding behind a wall.  Rip-Tires can be effective on D.va as long as her Boosters are still on a cooldown.  Her Fusion Cannons also work well for shooting down Rip-Tires.

Keep in mind that there is a short delay right when D.va ejects from her mech, this gives Junkrat a small amount of time to throw down a Trap/Mine combo to trap her when she lands.  Constant grenade spam can also kill a D.va ejecting from her mech.  Also keep in mind that a Mine does NOT knock back a Self-Destruct.

Pharah:  Pharah is by far the strongest counter to Junkrat.  Virtually all of Junkrat's weapons are ground based, which means Pharah is mostly in the clear with her air mobility.  A good Junkrat player might be able to hit Pharah in the air with his frag launcher, but it's tough.  Junkrat's mines and Rip-Tires are practically useless against Pharah.  However Mines can be used against Pharah by throwing them in the air to possibly deal some damage to her.  Pharah can use a Concussive Blast to propel herself away from an approaching Rip-Tire.

Pharah's ultimate Barrage can be effective against Junkrat, but if you catch it in time you can Mine jump away from it, or even attempt to get some easy frag grenade shots on her since she will be stationary while casting it.

Reaper:  Junkrat does a god job of blocking off choke points with his traps and Frag Launcher. Unfortunately, Reaper is able to Shadow Step around all that to get up close to Junkrat to take him out.

Reaper can avoid the Trap/Mine combo and escape out of the Steel Trap by quickly using Wraith Form, which could also avoid the mine explosion.  If you see a Reaper using Shadow-Step next to you, quickly throw down a Trap/Mine so he will take damage right after porting in.  Although if he is quick enough, Reaper can escape some of it with Wraith Form.

Since Reaper will tend to kill Junkrat close up, he will have to remember to always avoid Junkrat's suicide bombs when Junkrat dies, as he is very susceptible to them.

Rip-Tires can be avoided if Reaper can use a Wraith Form or Shadow Step to save himself from the blast.

Widowmaker:  Widowmaker is virtually safe from Junkrat's traps and mines since she is always way in the back sniping from a distance.

Although Junkrat can take cover while lobbing grenades, he is usually out of reach to aim his Frag Launcher at Widowmaker, and if Junkrat peeks in view, it's GG for Widowmaker.  Widowmaker's Infra-Sight only makes it easier for her to see when a Junkrat pops into view for her to shoot down.

Since Widowmaker is usually far away from Junkrat and can get up high with her Grappling Hook, she is usually safe from Junkrat's Rip-Tire.  Luckily, Junkrat usually doesn't need to worry about coming in contact with Widowmaker's Venom Mines since they are mostly used to keep offensive enemies from pushing toward her team.

Mercy:  Since Mercy is normally in the back taking cover while healing and buffing her team, she is usually out of sight from her enemies, but Junkrat can spam his grenades her general direction potentially being able to take her out.  Although sometimes you will find Mercy flying around in the air supporting heroes such as Pharah, in these situations it will be incredible difficult to take her out as Junkrat.

Since Mercy follows behind her teammates, she will rarely ever run into Junkrat's traps or mines, and will be able to heal up damage from allies that falls prey to the traps.

For Junkrat's Rip-Tire, she can use her Guardian Angel ability (on an ally) to propel herself away from the incoming tire.  Otherwise she will have to whip out her pistol real quick to try and shoot it down.

There is currently a big debate during beta with Junkrat's grenade spam and Mercy healing all the damage from the spam.  The problem is that Mercy gains ultimate charge while healing her allies.  This means that Junkrat's spam is essentially enabling her to reach her ultimate Resurrect faster and more often when healing up Junkrat grenade spam.  This is in current debate and could change in a future patch.  This is currently why I have her listed as a counter to Junkrat.

After Mercy uses her Resurrect ability, you will see the enemies getting resurrected slowly, that would be a good time to throw down a Trap/Mine underneath one of the resurrecting heroes to deal massive damage to one of them.

Soldier 76:  Soldier 76's Biotic Field is a good counter to Junkrat's Frag Launcher spam, since he and his team can simply heal up any grenade spam damage.  Soldier 76's Pulse Rifle works good on Junkrat at all ranges as long as he can get within view of Junkrat.  You might need to sprint to certain spots to get him within view.

Traps/Mines can trap Soldier 76, especially when he is sprinting.  This doesn't happen too often but sprinting can make Soldier76 overlook Traps/Mines before it's often too late to shoot at.  However, Sprint does help Soldier 76 tremendously with escaping Junkrat's Rip-Tires.

Soldier 76's Helix rockets and ultimate Tactical Visor also pose threats to Junkrat as well.  If you hear Soldier 76 use Tactical Visor, simply mine jump away to escape out of his view.

Zenyatta:  Since Zenyatta is a support hero that usually just stays in the back and takes cover, he normally doesn't need to worry about running into Junkrat's traps or mines.  His slow mobility further makes it easier for him to be cautious about running into his traps and mines, giving him enough time to spot and shoot at them.  Zenyatta is more prone to taking damage from Junkrat's Frag Launcher.

Junkrat also is safe from Zenyatta in the same fashion that Zenyatta is safe from Junkrat.  Since Junkrat is also in the back taking cover lobbing grenades, he is usually safe from taking further damage from Zenyatta's Discord Orb, and it's usually a waste of a Discord Orb for Zenyatta as he would be better off putting it on someone else.

If Junkrat receives the Discord Orb, he can use a mine jump to quickly get behind a wall to remove the Discord Orb.  Plus his nature of lobbing grenades behind walls also gives him more of a reason to take cover if he gets the Discord Orb.  Another reason to get behind a wall and lose sight of Zenyatta is for Junkrat to use his Rip-Tire ultimate.

Due to Zenyatta's poor mobility, Junkrat's Rip-Tire can be a huge threat to him.  If his ultimate Transcendence is not available to save him, he will most likely die to the tire unless it gets shot down before it reaches to him.

Genji:  Close match up. Junkrat will stay around walls/corners while lobbing grenades and hiding traps/mines for the approaching Genji.  Genji can deflect Junkrat's frag grenades and Concussive Mines, but not Traps.

As Junkrat, try to keep your distance from Genji and put traps/mines in spots where you feel he might come across.  If he gets close to you, try and mine jump away from him and towards your team so they have your back.  If you hear Genji use his ultimate Dragonblade, quickly throw down a trap (if you can) and then mine jump away from it.

Keep in mind that Genji's Swift Strike does not damage mines and traps, in fact a Steel Trap will still trap a swift striking Genji.

Genji can counter Rip-Tire by Swift Striking or climbing away from it.

Ana:  Ana will most likely have to watch out for Junkrat's random grenade spam, which can sometime be difficult due to Ana's poor mobility.  Ana may need to use more Biotic Grenade self-heals to deal with that.  A lot of times it will be difficult for Ana to have Junkrat within view for Biotic Rifle shots since Junkrat is able to lob his grenades while taking cover behind walls or objects.  This, along with Junkrat's random movements with Mine jumping also keeps Junkrat a bit safer from Ana's Sleep Darts and Biotic Grenades.

Since Ana is a backline support sniper, she is usually safe from Junkrat's Traps and Mines, but she should still watch out for them and kill them when scoped in closer to the enemy.

Due to Ana's poor mobility she is more susceptible to Junkrat's Rip-Tire and she would have to start moving away from it as soon as she hears it coming in order to avoid it in time.  Also if Ana spots Junkrat standing still while he controls his Rip-Tire, there is no need for Ana to use a Sleep Dart on him during that time as Junkrat will still be able to control his Rip-Tire while Junkrat is sleeping.  Instead, Ana should just try to take him out with Rifle shots and Biotic Grenades.

A Nano Boosted enemy (Ana's ultimate) is not a huge threat to Junkrat as you can simply mine jump away from it or use a Trap to hold the Nano Boosted enemy down for a short while.  However a Nano Boosted enemy may be able to effectively run away from Junkrat's Rip-Tire easier.

Winston:  Winston can block Junkrat's grenade spam with his Barrier Projector shield, protecting his team, only temporarily though.  Winston's Jump Pack can potentially jump over Junkrat's Traps/Mines and if Winston gets close to Junkrat he can use his ultimate Primal Rage.  Keep in mind that Winston's Barrier Field will block everything Junkrat has, including:  mines, traps and frag grenades.

To counter Primal Rage, quickly throw down a trap (if you can) and then mine jump away from it.

Rip-Tires can be blocked by a barrier shield, but they can be steered right inside of a barrier and then be detonated.  Winston doesn't have a long range weapon to shoot it with, although Winston can just leap away from it (if it's not on a cooldown).  Winston can also counter a Rip-Tire with Primal Rage to take the hit from it, otherwise one Rip-Tire will be able to knock regular Winston out at full normal HP.

Junkrat:  Vs another Junkrat it comes down to who has better skill at staying elevated higher with mine jumps, since that makes it harder for the opposing Junkrat to aim his Frag Launcher at.  Staying elevated also protects you from getting hit by Rip-Tires and Steel Traps.  Clever use of Steel Traps also come into play especially when close encounters happen.

Lucio:  Lucio's Crossfade heal aura is a powerful counter to Junkrat's grenade spam, but his speed aura can get him and his teammates trapped easier by Junkrats Trap/Mine combo, since faster movement makes it harder to dodge traps before enemies run over them.  Because of this, a smart Lucio player will use his heal aura more often than speed when facing an enemy Junkrat.  Although, Rip-Tires can force Lucio to use his speed aura to escape from the tire explosion, which could potentially make his team run into his traps easier.

Rip-Tires can be countered by Lucio's Sound Barrier ultimate.  If you just launched a Rip-Tire and an enemy Lucio uses Sound Barrier, then try and wait at least a few seconds before detonating the tire so their shields won't absorb all the damage from it.  Remember that Rip-Tire has a 10 second duration while Sound Barrier has a 6 second duration (shields will degrade 500 over 6 seconds).  You can snug tires up against a wall to pass the time until detonation.

Along with Lucio's heal aura, his Amp it Up ability along with his ultimate Sound Barrier, will make dealing with Lucio a tough job for Junkrat, as there is little he can do about those abilities.

Hanzo:  Since Hanzo doesn't have any super fast movement abilities he is less prone to getting trapped by Junkrat's Steel Trap or Concussion Mine, even though it can happen.  Since Hanzo can climb walls, he is able to get to spots where he is generally safer from Junkrat's grenade spam and more protected from Rip-Tires as well.

Hanzo can be deadly close up with his Scatter Arrow shots to the ground in front of you.  If you see Hanzo approaching you, simply mine jump away from him to take cover from him.  Mine jumps can also be used to escape from his ultimate Dragonstrike.

Another deadly ability that Hanzo can do is use a Sonic Arrow to spot Junkrat spamming grenades behind a wall or obstacle, Hanzo can then attempt to shoot a Scatter Arrow to ricochet in his area to knock Junkrat out.

Zarya:  Junkrat's mines and traps are no real threat to Zarya since she moves slow and is able to spot them before she runs over them, and even if she actually does run over them, she can quickly use her Particle Barrier to save her from any mine detonations.  If she is quick enough, she can save her teammates from the trap/mine combo by giving them a Particle Barrier as well.  Zarya can still get stuck in a Trap even with a Particle Barrier on, but it will transfer the damage to make her gun stronger.

Zarya is most prone to Junkrat's grenade spam, since she doesn't have any means of healing herself, besides her regenerating shield.  However, Zarya's alternate fire (explosive charges) actually tends to be quite effective on Junkrat, since it does splash damage and can hurt Junkrat hiding around an object or wall.

As Junkrat, if you know the enemy has a Zarya on their team, try to spread out further in battle by using your mine jumps to get to other random spots.  By doing so, you will be more protected from her ultimate Graviton Surge (which clumps up her enemies together in a ball).

Rip-Tire can be effective against Zarya, but she can shield herself (or an ally) from it with her Particle Barriers.  Rip-Tire has a decent duration for it, so you can stall some time while her barriers wear off.  Watch out for Gravition Surges, as Rip-Tires can still get stuck in them.

McCree:  This is a close match.  Junkrat will try to stay around walls/corners while lobbing grenades and hiding traps/mines for the approaching McCree. If McCree can mange to get up close to Junkrat while avoiding his traps/mines, it's GG for McCree. Since McCree's mobility is mostly slow paced, he walks around the map slow enough to see traps before he gets to them (unlike fast heroes such as Tracer or Genji), and can then shoot/kill them more safely.

Since McCrees will tend to kill Junkrats close up, they are very susceptible to Junkrat's Total Mayhem death bombs.  McCree will have the upper hand at medium range as long as Junkrat is within view.  Getting up close to Junkrat will still be difficult for McCree, and Junkrat has superior mobility with his mine jumps enabling him to escape away from him.

McCrees can deal with Rip-Tires by using a Combat Roll away from it while attempting to shoot down the tire, all this can be a little difficult for McCree.