Bastion:  Bastion's worst nightmare is being flanked from behind, and who excels at flanking the best?  Reapers!  Reapers are a great counter to Bastions, thanks to his Shadow Step ability.  Simply Shadow Step out of the Bastion's line of sight to get behind him to finish him off, then use Wraith Form to escape from the rest of the enemy fire.  Remember that a Bastion Sentry takes double damage from the rear.  If he turns around and starts firing at you, simply walk through him and continue firing.  Most likely Bastion will be too confused on where to turn.

Thanks to Reaper's Shadow Step ability, he is also a good counter for an elevated Bastion due to Mei's Ice Wall boost.

Mercy:  It's generally Reaper's job to take out Mercy.  Reapers tend to have the easiest time getting the closest to Mercy.  Work your way around the enemy and focus on taking Mercy out first if you can.  Death Blossom comes in handy especially when Mercy is around multiple allies.

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

TIP:  When Someone hits a Junkrat's Steel Trap, there is usually a Concussion Mine next to it waiting to be triggered to kill the one being trapped by the Steel Trap.  Reaper can avoid the Mine and escape out of the Steel Trap by quickly using Wraith Form.  If you got stuck in a Steep Trap and you were unsure if there was a Mine underneath of you, use Shadow Wraith anyways to potentially save yourself.

Junkrat's ultimate Rip-Tire can be easily countered by using Shadow Wraith, or even Shadow Step as a backup counter.  Shooting Rip-Tires down with shotguns tends to be very difficult due to the poor damage from medium to long range.

Also since Reaper will tend to kill Junkrats close up, you have to remember to always avoid Junkrat's suicide bombs when he dies.

Widowmaker:  Widowmaker is a long range sniper and can be hard to deal with sometimes.  It is generally Reaper's job to hunt her down (along with Tracer and Genji).  Reaper's sneaky mobility allows him to get around the map and kill Widowmaker unnoticed.  You must watch out for Widowmaker's Venom mines which are usually placed at choke points covering Widowmaker to specifically stop enemies such as Reaper.  If you can't avoid it, then use Shadow Step again to get around it.  If you get triggered by a Venom Mine, simply use Wraith Form to dispel it from you.

One must be very careful using Shadow Step around a Widowmaker because you may find yourself getting shot in the head immediately after Shadow Stepping if a Widowmaker spots it.  Try to use it at a destination spot that is not in her line of sight.

Winston:  Winston mostly relies on his teammates for cover, but if he wonders off alone he may come face to face with a deadly Reaper.  The best thing for Winston to do is simply retreat back to his teammates with his Jump Pack ability, or take a chance and turn on his ultimate Primal Rage, but Reaper can simply use Wraith Form to escape it.

Remember when you are inside of Winston's Barrier Projector you cannot shoot at enemies that are outside of the barrier, as the barrier works to block shots from either side (in or out of it).

Winston can counter Deathblossom by using a Barrier Projector, which could potentially protect his teammates as well.  Winston can also use a Jump Pack leap to avoid further damage.

Symmetra:  Symmetra is not much of a problem for Reaper.  The worst thing Reaper needs to watch out for is her Sentry Turrets.  Since Reapers are always moving around hallways/passages, Sentry Turrets are usually placed in these areas.  Reaper's shotguns are okay at killing them quick enough, and you can always use Wraith Form to further protect yourself from dying to the turrets if you need to.  You can also Shadow Step past rooms filled with turrets.

It is also Reaper's job to go and hunt down Symmetra's Teleporter ultimate due to his superior mobility.

If you manage to get close to Symmetra there is not much she can do to you and her poor mobility will make it difficult for her to escape your Death Blossom.

Ana:  Ana's poor mobility and low damage output makes her very susceptible to getting flanked by Reaper's deadly close range shotguns.  As Reaper, the main thing to watch out for is Ana's Sleep Darts.  Unfortunately, Reaper is one of the few offensive flankers that is easier for Ana to aim Sleep Darts at, as Reaper doesn't have quick movement abilities like Tracer and Genji.  If Ana misses her Sleep Dart it can be an easy kill for Reaper.

Ana's next best option to use against Reaper is Biotic Grenades, which will do some damage to Reaper while also preventing him from getting healed or preventing him from picking up Soul Globes.

Ana's poor mobility makes her very vulnerable to Reaper's Deathblossom and she will most likely die if caught in the midst of it.  Once again the best thing for Ana to do against Deathblossom is to quickly take Reaper out with a Sleep Dart before he can do any damage.

Be careful flanking one of Ana's allies, as she can use her ultimate Nano Boost on that enemy to make them powerful enough to either escape from you or hunt you down with the increased damage reduction and damage output.  In this case, it might be wise to run away from a Nano Boosted enemy by using Wraith Form.

Mei:  Although Mei has a great short range freeze weapon, it usually isn't enough to stop a Reaper.  Reapers simply dish out more DPS and have more health than Mei has.  Reaper can use Wraith Form to avoid getting frozen and use Shadow Step to get around Mei's Ice Wall to take her out.

If you see yourself getting stuck in Mei's ultimate (Blizzard), simply use Wraith Form to escape it.  A quick Shadow Step is another way to get out of a Blizzard as well.

Mei can counter Deathblossom by quickly using a Cryo-Freeze or an Ice Wall to block it.

Torbjorn:  Reapers tend to do very well at killing defensive heroes, and in the case of Torbjorns it's no different.  However, Torbjon's turret does make it a bit harder for Reaper to deal with.  If you can, use Shadow Step to avoid Torbjorn's turret but still able to shoot at Torbjorn himself.  If Torbjorn uses his ultimate (Molten Core) and he is repairing his turret, I find it's practically useless attacking the turret since he will be able to repair it so quickly.  Instead, I find it more effective at attacking the Molten Core Torbjorn himself in this case, just try and take cover from the turret in the meantime.

Torbjorn can counter a Deathblossom by using Molten Core to heal up and gain a large amount of health to withstand it.

Pharah:  Pharah is the best hero at staying from harms way of a Reaper.  Her jet packs allow her to quickly get high above in the air to avoid shots from a Reaper.  When she lands on the ground she can use a Concussive Blast to propel Reaper away from her.

TIP:  If you can, use Shadow Step to get high up on ledges around the map where Pharah may not be looking, you may be able to get some decent shots off on her.

Pharah can easily escape from Reaper's Death Blossom by simply using her Jet Packs to fly in the air to avoid it, or by using a Concussive Blast to knock Reaper away from her (or from her allies).

Reaper can avoid Pharah's ultimate Barrage by using Shadow Wraith, if it's not available, then Shadow Step can be a backup counter for it.

McCree:  Both McCree and Reaper are two of the best close range heroes in Overwatch.  But who wins?  McCree mostly has the upper hand here thanks to his Flashbangs.  If Flashbangs are used faster than a Reaper can use Wraith Form (Wraith Form is immune to Flashbangs), then it is GG for McCree.  Flashbangs also interrupt Reaper's ultimate (Death Blossom), totally canceling it out.

It is best to only face a McCree if you are flanking him from behind.  Once he turns around and looks at you, I would immediately use Wraith Form to escape, unless you are for certain his Flashbang is still on a cooldown.

If you see McCree using his ultimate Deadeye (High Noon) and you don't have enough time to lose line of sight from it, then Wraith Form is the best escape from it.  Shadow Step may not give you enough time to avoid it.

Lucio:  All of Lucio's abilities are a threat to Reaper.  Lucio can use his Crossfade speed/heal aura on his whole team to help them avoid Reaper and his close up shotguns.  Lucio can use Soundwave to knock back approaching Reapers and his ultimate (Sound Barrier) protects him and his whole team from Death Blossom.

When Lucio uses Sound Barrier, it's usually best to not engage in combat for a few seconds as it's virtually pointless attacking the high shields that his allies can receive from it.  This would be a good time to reposition yourself elsewhere to confuse the enemy of your whereabouts.

Hanzo:  A Hanzo should be one of Reaper's prime targets.  Dealing with a Hanzo is similar to dealing with a Widowmaker, Reaper can sneak around the map and take Hanzo by surprise.  Hanzo has very little escape methods (besides wall climbing) so a close up Reaper is very deadly to Hanzo.

Be careful using Shadow Step around Hanzo as he can headshot kill you in one hit after teleporting in.  Hanzo can also use Sonic Arrows to detect offensive flankers such as Reaper sneaking up on him.

When coming up close to a Hanzo, he can kill Reaper with a Scatter Arrow shot to the ground in front of you.  You can kind of avoid this by jumping continuously when you get near him.

When Hanzo uses his ultimate Dragonstrike and you find yourself caught it the midst of it, you can use a Wraith Form to escape it safely.

D.va:  Even though Reaper out DPSes D.va at close range, she still has enough hitpoints to take a number of hits from Reaper.  Since both heroes have great escape mechanisms, it usually doesn't result in a kill from either hero most of the time.  D.va can use her Boosters to boost away from Reaper, protecting her from not only Reaper's shotguns but from Deathblossom as well.  D.va's Defense Matrix can also block Death Blossom too, potentially protecting her whole team from it.  While Reaper can use Wraith Form to escape from D.va or use Shadow Step to quickly get right up close to D.va which can be very deadly to her.

D.va's ultimate (Self-Destruct) can be easily avoided with Wraith Form or Shadow Step.

Zarya:  Reapers tend to avoid coming in contact with Zaryas, since she is usually with her team absorbing damage.  Zarya can avoid being attacked with her Particle Barrier and it can take a number of shots to take her down.

Reapers are usually in the clear of Zarya's ultimate (Graviton Surge), since Reaper is usually not around his team most of the time.  If Reaper does happen to get caught in Zarya's Graviton Surge, he can simply escape out of it with Wraith Form.

Death Blossom can be deadly to Zarya if she cannot use a Particle Barrier to protect herself from it, as her slow mobility will easily get her stuck in it.

Roadhog:  Being chained in by Roadhog can be very deadly to Reaper.  Wraith Form is the best way to save yourself after getting chained in.  If Wraith Form is still on a cooldown there will be a good chance to die to Roadhog's Chain Hook.  As Reaper you are better off keeping your distance from Roadhog and keep strafing around to avoid being chained in.  You can still do decent damage from a distance on Roadhog thanks to his large hitbox.

Be careful using Death Blossom around a Roadhog because he can use his Chain Hook on you to cancel it out.  Try to time your Deathblossom so you execute it when Roadhog's Chain Hook is on a cooldown to avoid it.  Roadhog can also use his ultimate Whole Hog to knock a Deathblossom away from his allies.

Roadhog's Whole Hog can be counter by using Wraith Form.

Genji:  Going up against a Genji is somewhat of a fair fight. Both heroes have ways to escape out of combat. Genji will still want to keep his distance from Reaper (his climb ability helps him with that).  Genji's Deflect does work well against Reapers shotguns, and you will want to avoid shooting a Genji with Deflect on.

Genji's Deflect is devastating to Reaper.  If you fire at Genji with his Deflect on, he will be able to instantly transfer the damage right back to you, due to the instant hitscan nature of Reaper's shotguns.  This makes using Deathblossom around Genji extremely devastating to Reaper.  If a Genji sees a Reaper using Deathblossom, he can Swift Strike right up to you and then activate his Deflect and it will usually kill Reaper very fast.

Genji's ultimate Dragonblade can be countered with Shadow Wraith, or Shadow Step if you can do it in time.  Remember that Genji is very vulnerable while using Dragonblade, so a few shotgun shots can still take him out quickly.  But be careful because Genji can still use Deflect while Dragonblade is active.

Tracer:  Both Reaper and Tracer have similar roles they pursue (sneak around and flank).  But when they come head to head with each other, it all comes down to who has quicker use of their abilities.  Tracer will need to use Recall after only one good hit from Reaper in order to refill her life, and she will need to use Blinks very strategically to avoid taking any shots from Reaper.

The main thing Reaper needs to watch out for is when Tracer uses her ultimate (Pulse Bomb).  As soon as you see Tracer lob out her Pulse Bomb at you, quickly use Shadow Wraith to counter it.

Deathblossom can kill Tracer quickly, unless she responds quicker by blinking or recalling away from it.

Reaper:  Reapers rarely run into each other because they are off flanking the enemy, but if they come in contact with each other it comes down to who gets their shots off first and correctly escaping out of combat when low on health.  A lot of 1on1 Reaper fights usually doesn't result in a kill, because of their escape via Wraith Form.

An enemy Deathblossom can be easily avoided by quickly using Shadow Wraith.

Soldier 76:  Soldier 76 usually sticks around his team to support them with DPS and healing and doesn't usually come in contact with Reapers 1on1 a whole lot.  Reapers can attempt to flank Soldier 76 from behind which can result in a kill.  However, Soldier 76 can sprint away from a Reaper to avoid damage.

Soldier 76's ultimate Tactical Visor can be deadly to Reaper as long as you can't take cover from it or activate Wraith Form to save yourself from it.

Zenyatta:  Zenyatta's Discord Orb is what makes going 1on1 with a Reaper somewhat difficult.  Zenyatta is a prime flanking target (like Mercy), and should be high on Reaper's list of enemies to kill first.  Zenyatta has very little escape abilities (besides his ultimate Transcendence), so Reaper should have the upper hand at killing Zenyatta quick enough before his Discord Orb does any amount of damage to you.  Even if you do receive the Discord Orb you can remove it by simply using Wraith Form.  Reaper can also use Shadow Step to quickly get behind a wall to remove the Discord Orb if he is far away from one.

Reinhardt:  Reapers rarely encounter a Reinhardt 1on1, and when they do it can be quite scary for a Reaper.  Reapers need to be close up to deal good damage to enemies, but with Reinhardt, you don't want to get too close to him or he will either use his strong melee attack (Rocket Hammer), or Charge at you to attempt to pin you.

Reaper needs to be extra careful around Reinhardt, keep a greater distance than usual and you will be fine.  If you can, Shadow Step to a higher ground where Reinhardt is at and shoot down with your shotguns.  Reinhardt's weakness is lack of vertical mobility and there is not much he can do if an enemy is above him besides attempting to throw a Fire Strike, and you will be safe from his other Rocket Hammer attacks, his Charge ability and his ultimate Earthshatter as well.

Reinhardt's shield can be a strong counter to Deathblossom, by shielding his team from the shotgun massacre.