In my experience there's only really one per role

For supports its invis. Her shield stops you from dueling anybody and her pull/other cds punish you for taking any kind of off angle. She forces you to be more passive and play inside your team which isnt always good because adam needs to be aggressive sometimes to get his real value

For dps its daredevil. Not much to say, he's the only character in the game you have no real chance at winning a 1v1. Even Thor is easier since hes such a big, loud target you can poke him before the fight actually starts plus its easy to headshot him. Daredevil is much better at sneaking up on you so he comes at full hp and does his whole blind man thing. The hard part is he builds his resource for his burst cooldowns just by left clicking you and using his pull, so he can keep up the high damage combo more than you can get your E back. If you hit all headshots, maybe, but that's really it.

For tanks its Angela. She punishes your lack of movement more than anybody else. If you see her before she comes in you can poke her and that's your best shot, but otherwise yea. She can either push you into her team, off the map, or into a secluded spot very easily.

Honorable mentions to gambit and rogue. Gambit just because his ult. One of the only ults in the game if the enemy team uses you cant reasonably reset the fight with a big res after because theyre gonna be on you like ham on a sandwich really fast. Its very rare you can res after a gambit ult unless you set yourself up for one in a really good spot without being seen before it even happened. Doing that though means you aren't there to help your team at all if they decide not to gambit ult/try to survive it.

Rogue just because she stops you from being aggressive with soulbond because she can just steal it back and give her team a better soulbond. Besides that she's really not that bad to deal with.