DVA is OK. (Use her as she was intended -a flanking tank)
Winston can use a shield CD buff. (Otherwise he is ok)
Junkrat is a bad pick because he is countered by half of the heros. (He is an OK pick under gold.)
Pharah is not a bad pick against 76 if you're smart. (Learn to use your E to get out of dodge)
Mercy is an OK pick if you can't play other supports. (The reason why pros don't picks her is that ana is stronger and LĂșcio speed boost is way too valuable.)
Symmetra has a high reward despite her high skill floor. She is not OP but shouldn't be that rewarding. A small nerf like making her gun not charging on shields should be enough. (Keep 7m distance from her and use a long range hero to neutralize her, also think before you move)