Since Genji is a great flanker, I love using him as a great counter for Ana to not only deal damage, but to also dodge her sleep dart and biotic grenade.
Using his Fan Blades at close range, I have used Swift Strike against Ana for the ultimate counter and finish her defeat. Genji's Deflect is also a great counter to Ana's biotic grenade, bouncing it right back to her and dealing damage.
If Genji is a bit of a shaky hero for you, Tracer is great to counter Ana as her mobility is much more reliable. Although her primary attack is not as powerful, she can outsmart Ana very quickly. As someone who plays with Ana as a support main, I can tell you the Tracer is one of the hardest counters for Ana. Especially when Tracer uses her Blink ability.
Something to keep in mind, though, is that Tracer doesn't have an effective defense against the sleep dart or the biotic grenade. Only counter Ana when you have learned the ropes of Tracer's movements. If you master Tracer, you can exhaust Ana really quickly, and give her no option but defeat. This is why I would definitely go for Tracer to kill Ana and create a good defense matrix.
Kiriko is one of those new kids on the block, making herself worthy being great as an Ana counter. What makes Kiriko a good counter, though, is not only because of her Protection Suzu, but also being a solid support hero.
Kiriko offers an awesome method to heal her teammates that counters really well against Ana which only a few heroes can perform. If Ana is ever with a good Lucio, Kiriko will be your go-to support to counter Ana and Lucio together. With Kiriko's Swift Step and Protection, Ana has no chance.
Reaper's effective range really put a dent to Ana's primary fire and combat methods. I personally love to use Reaper as my all around flanker, especially for Ana counters as he has a really high damage output when flanking properly.
With Reaper, you have to be really careful as he can get easily spotted if you dont know how to play with him. However, he is one of a few heroes who can perform melee attacks at a close range without being affected by Ana's sleep dart or grenades. Especially if he can escape using his wraith form, you're practically toast!
Reinhardt is a beast, and he also is a good counter against Ana. Although he cannot defeat Ana alone, his shield can thoroughly protect his teammates from Ana's sleep dart and grenades to give them the upper hand.
Since he is a really large unit, I learned he is a too big to be a flanker, although I have used him to give major advantage to damage heroes to provide much better close range combat with Ana. This way, she has no way around his Barrier Field, putting her in a really poor position not only without secondary weapons, but horrific range for her biotic rifle.
Although she is not the number 1 best counter for Ana Overwatch 2 has given, Symmetra has some great counters that will most certainly deal damage. Given her Photon Barrier and Projector, she is bound to being on the list of ana counters.
As one of the Ana counters, Symmetra can stop her biotic rifle shots rather quickly, along with protecting her team from being in range to her sleep dart and grenade. Combined with the Barrier, the Photon Projector makes Symmetra's hit box extremely small and annoying to target. I have failed so many times as an Ana player against Symmetra for this specific reason. Symmetra is hard to counter!
Although Sombra is last, she is definitely not the worst of the top 7 Ana counters listed. Sombra is a beast to counter, and since she has fantastic abilities to hack Ana's ability options, this greatly increases your teams chance of victory.
Sombra can not only flank well, but if she hacks Ana and flanks her at the same time, Ana has no way to counter back without her sleep dart and grenade. She has no other choice but her rifle, which even if I tried to use it for a quick scope, I won't have a chance against Sombra.