Which hero or heroes do you see on the other side and just hope there's not a halfway decent player behind them? Who seems to have an innate capacity to dodge your bullets and always score hits against you with theirs; regardless of which character you pick?
For me it's Junkrat and Genji. They drive me nuts.
Poll only has 20 options, had to put two together so it's Zenyatta and Symmetra because I just refuse to believe anyone finds them a real issue :3