Like with any tank class. You most likely aren't going to take them down 1v1, at least not if you are within range of their abilities
As others have said, if Roadhog's hook is on cooldown that is your optimal time to engage. Yes, he has a heal but doing damage to him while he heals eats up what he is trying to gain back. He is also stationary and can't return fire while healing. People often die to RH because they forget that his hook timer will come back up and they try to get a little extra damage in only to get hooked and 1 shot
Pharah, Genji, Reaper, Bastion are all direct counters.
Pharah, sadly, needs some love of her own at the moment.
Yesterday's patch made McCree even more fun to play. Increased ROF on his alternate made you able to unload your clip then slide more effectively before the stun wore off.