D Va is by far the best counter for Reaper in Overwatch 2. Using her Defense Matrix this can stall Reaper from doing any damage to his enemies. Along with D. Va's fast maneuvering abilities, this really saves her from possible death from Reaper's flank or ultimate.
As someone who mains Reaper (because he's the best Damage hero, duh), I have spotted many players switching to D. Va specifically for her Defense Matrix. It is the best counter for Reaper's Hellfire Shotguns and Death Blossom, blocking any and all damage that could be possibly inflicted.
With Reaper's Ultimate, I have countlessly lost it due to Zenyatta's Transcendence. Zen's ultimate is probably the fastest counter for Reaper that I have ever seen. If you are playing Reaper, I highly advice you use your Ultimate wisely when Zenyatta is around to avoid possible cancel's.
Zenyatta not only has a great counter with his Ultimate, but to counter Reaper his Orb of Discord will greatly benefit the teammates to quickly bring him down. Whether Reaper is flanking or in front of the push, Zenyatta's Orbs will greatly counter Reaper to his defeat.
Since Reaper has a rather high damage absorption, Hanzo's arrows cause the perfect amount of damage to keep Reaper away from causing close range damage. I have also been caught flanking many times by Hanzo when he is using his Sonic Arrow to track my moves.
Hanzo, though, does not counter Reaper well when it comes to his ultimate. Since Hanzo is mostly aggressive hero play, he relies on support heroes to defend him during incoming attacks from the enemy. Overall, Hanzo offers fast, effective strategies to take Reaper down quickly.
Since Echo is an air hero, she can easily get a better visual as to where Reaper is going. Along with this, Echo has time and time again cancelled my Shadow Step and Wraith Form due to her sticky bombs. If you can counter Reaper quickly with these bombs, you are sure to bring him down.
If you aren't too familiar with Echo, dont worry! She is great at countering Reaper for even the average ranks in Overwatch 2. However watch your back, because if Reaper does flank successfully, your Flight ability might be the only thing that can save you.
Sigma is another hero who really annoys any Reaper player, including myself. Sigma's abilities keep him at arms length from Reaper which greatly increases his chances of avoiding Reapers close range damage.
In most games I play against Sigma, when he counters me I can never get a good shot on him, especially in 1v1 situations. If I flank Sigma, he can immediately turn around and deploy his barrier that immediately deflects any close range damage I can inflict as the enemy Reaper. This along with his Accretion ability, you sure to keep Reaper's abilities at sub-par.
Although Ana is one of the worst healers in Overwatch 2, her tactics are perfect to distract and keep Reaper from damaging her allies. As this sniper can be great Reaper counter, I have many times found myself in immediate defeat if I am down her sights.
Although Reaper can counter Ana really well if he is close enough, at long distance he just has no chance against her sniper and sleep dart. These abilities immediately put Reaper out of the equation and will boop Reaper back to respawn.
Orisa has great defensive tactics to counter Reaper in Overwatch. With her Javelin Spin, Reaper has no chance but to use his wraith form to avoid possible damage from nearby enemies. I have found myself trapped many times with Orisa using Reaper, and Shadow Step has quickly become my best friend in bad situations like this.
Orisa's Javelin Spear is also a great counter for Reaper on the enemy team if you are experiencing issues with his flanks. I have found myself spearing Reaper multiple times, resulting in a quick death due to its high damage output.
There are some great Pharah counters out there, but she makes sure to be one of the best heroes to counter Reaper, die trying! Anytime I hear a large explosion I already know who's coming for me, as Pharah's Jet Pack gives her the best angle to shoot her grenades.
However, I have found myself flanking Pharah quite frequently, which result her to a pretty rapid death if she doesn't dash out of the scene quickly. Learning how to use Pharah before using her as a Reaper counter is probably your best option in this case.