It's absolutely whatever character you're best with. There are 2 top 500 torbs that I know of that can easily carry a master or diamond game by themselves. I've seen carry anas, carry widows, carry hanzos, carry lucios (go watch dspstanky absolutely shit stomp diamonds and tear apart master players), and carry pretty much everything else.
It's this simple: If the other team has 6 scrubs and your team only has 5 scrubs you're going to win. If you didn't win then you must have also had 6 scrubs. It's not about the character. It's about understanding the game on a higher level than you currently do. 99% of this game is positioning and situational awareness that's why people who think they have good mechanics (and they might) end up stuck at some low SR. If you can't climb I can almost guarantee it's an issue with your tactics and not your mechanics. Go watch a bunch of Unit Lost's videos on youtube. He has videos explaining the major mistakes players make at each skill tier (bronze, silver, gold, plat, diamond, master, gm).