Best bet is to pick one from each category and be good with them. That way you can flex about your pick to suit the team which gives you a much better chance of winning when playing with randoms.
Soldier is a staple DPS to begin with but doesn't have the ability to carry like a really good Pharah does.
Rein is always a good tank to have.
Fucking Hot Healing Chick! is still great post nerf. Just remember when you're playing support, the aim is the help your team and not worry about killing the enemy team. So I wouldn't get sidetracked by Lucio'so damage
Mei is situational I find and sometimes just doesn't add enough to the team comp.
Junkrat is ace after his recent extra mine. Makes him a lot more scary up close.
I'd suggest playing less characters but getting really good with a few. It'll really help get you through the lower comp levels. At the SR you are at the moment, you can totally solo carry a game.