I play Widow and I don't think she's OP or anything, but people who are good at sniping in other games have an easy transition to using her, so the average skill of a Widowmaker is artificially inflated right now. The other heroes will catch up soon. Anyway, here are her counters:
Having a Tracer get behind the lines to forget her up is a great counter, and it's not very hard to execute IMO. If I can't countersnipe an enemy Widow, I know my Tracer will do the trick.
Pharah is also a good counter but if she forgets up she's dead meat basically. Very volatile counter
DVA can literally just fly at her position with defense matrix up and force her to reposition or 1v1, but DVA wins because she has better DPS and health. (and she'll have even better DPS after the next patch supposedly)
Getting Genji behind the lines is a bit harder but he can also forget her up at close range. Same thing with McCree.
Characters like McCree who have accurate weapons can also take pot shots at her and actually stand a chance. I've won long range duels against Widows with McCree and Mei before.
I've had Soldiers force me out from good sniping positions and even kill me at long range. Helix rockets seem pretty effective.
Shields are also a great counter, she can't snipe through them. If you're pushing and having trouble with her, you just need a Rein or a Winston to use shields, and characters can force her out from her position from behind the shield. (most characters can do this to some degree)
If you're not able to do any of that for some reason, you can just countersnipe with Hanzo or Widow.
For Widow to get lots of meaty kills, there are very few positions where she is very safe but also has lots of access to where the enemy will be. The more open space she has available to snipe, the more space other players have to harass her with. On maps with chokepoints she can be particularly annoying (like the first chokepoint of Anubis is rough) but she's by no means invincible even there. It's all about forcing her to reposition and give up space or hard countering her with particular heroes. If you don't employ these counters, a good Widow will wipe the floor with your team though.
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