Raised my MMR ~500 within a week by playing mid and focusing on my own game.
4/5 of your teams mistakes are going to be made by other people. Accept and embrace it. Instead of spending your time watching what your team does wrong- redirect that focus to the enemy team. They'll be just as "dumb" and if you're truely a good player you will know how to exploit that.
Snowball heroes will always be the go to for raising MMR because of how much game impact they can have. GL!