Me. Well sometimes it's me, other times it's just matchmaking. I have looked at games after they have ended and realized that if you swapped one player over, the other team wins. I have had supports so bad that I literally couldn't do anything, I don't know if they were auto filled, or it's because I'm in Silver, but when you're playing Janna, standing max shield range behind me isn't how you are suppose to play that champ. I've had more supports than I can count decide that as far behind me as they can reasonably get away with standing is where they should be. Meanwhile the enemy support is moving up and harassing, the enemy ADC is trading, and I may as well be in a 1v2 lane for all the good my support is doing rubbing one out under turret. That or I get the sup that rushes full bore into a Lulu/Lucian comp and flames me when they get instantly blown up. Like, not sure what you wanted me to do, I'm playing Jinx bro, were level 3, that Lucian does more damage than I do, and you're Soraka, there was no winning that, enjoy the walk back. Or the wonderful, we have 4 minion to their 10, seems like a good time to flash in and flay them, try to weave a hook through all these minions who are beating my ass, then wonder why I died so fast. Then I get the spicy GG ADC does no damage. No bro, Minions just wooped that ass, and the enemy ADC helped a bit, again, not really sure what to tell you, I'm not getting involved in your awful decisions.
My favorites though are 2 very recent games I played, when I decided I couldn't handle the support lottery anymore, now granted, I'm not absolving my self of blame here, but when I have a top laner that just straight up abandons top lane because "I can't lane against Illaoi" I literally don't even know what I'm suppose to do. Maybe don't pick Yi top second pick? Maybe don't pick Yi top period? Maybe learn how to walk away from her, I dunno, all of the above? I was Level 5 on WW, got a good early gank, and got first drag after my first back. Dude just straight up abandons his lane, and just says he can't lane against her, so just leaves her to do what ever she wants. I don't remember what the other match-up was, but I very much remember this one.
I guess what I'm trying to say is there is no one thing, sometimes it's because I'm playing like an ape, other times it's because my mid laner is 0/9 at 7 mins, sometimes it's because my top laner has just decided to walk away, sometimes it's because bot lane doesn't know how to play against Zac and he ganks it repeatedly because they haven't figured out that pushing up to the enemy turret on Red side is probably a bad idea. If I had to put a quantifier on it, probably the worst player, because no matter who wins the game, the opposing team always seems to have one player that's made it there mission for the game to try to end world hunger, and at least in that regard, they generally tend to succeed.