Doomfist, because he has a stun + movement + damage ability on a 4 second cooldown. Not to mention it gets reset if he blocks 100 damage, but teammates charging his punch is annoying af. Also, he's painful for almost any support to deal with, both as an enemy and ally. For supports, unless you've got a clear slight on him in the enemy backline, you're only healing your doomfist if he retreats to get healing, or if you run to some point you can heal him from.
His saving grace is that his damage is fairly low, and he is easy to kill if you know what you're doing, but even then if it's a good doomfist then he'll know when to dive out, how to avoid cooldowns, to charge block consistently, etc... good dooms are a pain.