In comp I normally fill so I can play most of the characters that are needed in the current competitive meta. I normally end up on Zen, Lucio, or main tank. My Rein is passable but far from good, but my Winston is fairly decent(assuming the team is willing to play a comp that helps the Winston).
I wish I was better with most of the dps characters but since they're always instalocked its difficult to get some play time in an actual competitive environment. Genji and Tracer mostly as you can have an enormous impact on the game if you're really good with them.