I feel like the characters are generally pretty balanced. I'd say they all tend to have at least one hero who can quite easily counter them.

I'll use Bastion as an example as I've heard a lot of people say he's overpowered. He's fairly inaccurate when in turret mode so any long-range hero such as Hanzo or Widowmaker can easily take him out when in he's out in the open. If he's camping around a corner a fast character who can flank easily such as Tracer or Reaper can get behind him and do a lot of damage. If he's hiding in a building Pharah or Junkrat can quite easily killing him through their explosive damage.

If you want to win games in Overwatch you have to be able to adapt to situations and sometimes that means playing characters that you might not enjoy as much as others but I guess that's just part of the game.