I've found to my surprise that even picking your most played character, strapping on your best sunglasses and being all-around better than your team doesnt help when you're the last one alive from your team at every push/defense wave...
It's difficult to win alone, but here the aim is to make just enough of a difference.
In more practical terms I'd advise carries to play Soldier, Roadhog, Reaper, Pharah, Zarya and Tracer, generally in that order of preference depending on the map. Genji and Winston too, to a lesser extent.