I know I'm a broken record on this, but Widow really, REALLY needs her kit reworked. Like, who does she even counter? Who is she supposed to be good against? Zenyatta, I guess, he doesn't have many options against her at a distance. Torbjorn IF AND ONLY IF your out of his turret's MASSIVE range. Bastion if he doesn't already have his eye on you. Symmetra if, for some stupid reason, she's not hiding. And that's it.
Who can counter her?
Here's how each counter runs down if you don't get the hit from the first headshot.
Genji: Super agile with vertifcal movement, has dash, and reflect. Has every opportunity to evade another shot. Can easily flush Widow out, Widow can do little about it. He EASILY kills her.
McCree: His pistol can function as a sniper so if he ducks and weaves (something Widow can't do while scoped), he can outshoot her with a bit of good aim. Plus, dead eye of course. He has more trouble than most, but he still has a decent shot.
Pharah: theoretically, Widow is supposed to be a counter to her, but at best, she can shut down her ult pretty well because she is staying completely still there. Otherwise, she can fly away, has the concussion blast to fuck up your aim when her regular fire isn't killing you. A Pharah will win most of the time.
Reaper: He of course can ghost away to cover, but I've had instances where I saw him teleport next to me, and he out DPSed me even though he not only triggered my mine but also I fired on him with my assault rifle. The spread on her gun is VERY wide, so missing is easy. You might think "Well, he's teleporting, it should be obvious because of how he yells it out". Yeah, well, again, that scoped vision is a son of a bitch. I need to get out of it to turn around and get some situational awareness, and if I try to snipe him, the charge takes too long to get a clean headshot. That's assuming he just doesn't sneak up, which is probable, where he DEFINITELY will out DPS you, in case you just think I suck with her assault rifle.
Soldier 76: He has sprint to get away and can self heal and can even kill you pretty fast if he feathers the trigger. He can EASILY take out Widow, both from a distance and by flushing her out.
Tracer: Well, admittedly, she'll be pretty weak even after one body shot, but that's what revert is for, or blinking away to a health pack. Assuming that the player is reasonably familiar with the map, taking or smoking out a Widow is one of Tracer's easiest jobs.
Bastion and Torbjorn: If they picked a good spot, they can out DPS WIdow because of her charge requirement, so even the heroes she's good against is conditional. Remember, the Overwatch maps are small, so Widow is always likely to be within 40 meters. She only counters these guys if they have other problems to worry about.
Hanzo: I feel their are more or less matched in their sniping ability. He can't shoot as accurately from far away, but can fire off more shots much faster. Medium to close range, Widow is FUCKED. Even if his aiming is bad, his scattershot is WAY better than her poisonmine too and will mean the end of her often.
Junkrat: Another relatively easy prey for Widow, if only due to his slowness. Still, even he can use his explosive mine to escape if he's quickwitted enough. And, of course, if he is close enough to where his grenades can reach widow, she's gonna have a bad day.
Mei: Ice barrier, wall, long range alt fire. Like Junkrat, she's a bit slower so she's slightly easier prey, but she has many options for escape and atleast one for retaliation.
All the tanks: Their health almost ensures their survival. D.Va is pretty much her hard counter with her rockets. Winston is basically another one with his jump. Reinhard's Shield will protect him. Roadhog's Hook will get her as soon as he closes in even a little. Zarya is the only tank that is somewhat weak to her. She has no long range capabilities or movement. Her barrier will only help for 2 seconds. But like all tanks, she just has so much health, there's almost no way she won't atleast escape behind cover.
Ana: Ana's rate of fire means she'll put down Widow faster than Widow will put down her because of charge. She's less mobile, but she has a way better kit with a self healing grenade and a long shot sleep arrow.
Lucio: Super fast, self healing, and Ult that protects him, and has no fall off damage for his gun. I've played against plenty of Widows that I outshot as Lucio. Either they retreat or they die. I think WIdow is actually the only character that Lucio can make retreat.
Mercy: Situational, but guardian angel can easily bail her out and she has her pistol that can do what Lucio's and Meka-less D.Va can do: Fire on a stationary, scoped in widow that can't move without losing her charge.
Keep in mind that pretty much ALL the maps are designed around having LOTS of cover. There's very few sniper positions that both offer a good view and also aren't instantly identifiable and easy to flush out. Widow's only line of defense when she's scoped is the poison mine, which is borderline worthless. The sacrifice she makes in situational awareness can't be overstated, yet she has essentially no option to get her out of a jam. Her assault rifle is wide firing low damaging shit, especially when she's doesn't have the jump on someone else. She might be able to grapple away, but only once (for some ungoldy reason, it's got a TWELVE SECOND cooldown. Why?) and it's not likely to save you. Not even her Ult will necessarily save you if you are scoped in. And it's basically imperative that you are because if you aren't, you lose your charge.
There is no other hero as poorly equipped as Widow to deal with the vast majority of threats against her. You can argue that she's pretty good if you have godlike aiming that gets a headshot EVERY shot, but ANY margin of error is going to be disastrous for widow. It's not like that for any other hero. No other hero is this limited in their alternative options and escape solutions. If you're don't have the enemy down by the first shot, you're basically done or atleast no way you're getting that kill without other factors coming into play, like if they happen to be on cooldown or they are distracted by another enemy. Any other situation leads to them escaping, and more than likely taking you out. They have so many ways to do so, and Widow can't do anything about it.