Graves, because he's a god damn tank in every sense of the word. He's decently armored, he's carrying multiple people, and he has a very big gun.
Tristana, because rocket jump An Enemy Has Been Slain rocket jump Double Kill rocket jump Triple Kill rocket jump QUADRA KILL rocket jump PENTA KILL rocket jump LEGENDARY KILL rocket jump YOUR TEAM HAS DESTROYED A TURRET.
A game that promotes slavery and gambling under the illusion of 'friendship' and 'sportsmanship' is automatically an awesome game. - dydt
Vayne, Kog'Maw and Ezreal. The first two assuming you mean who can carry hardest hypothetically, and not who carries most often. They out carry based on their power and abilities, not necessarily leading in terms of win rates. Kog especially needs the rest of the team to not suck to utilize his power. If your team is crap then sub Kog for Graves.
Ez+Sona, Taric+Graves and Nunu+Vayne(or Kog) are the probably the safest bets when want to win lane and then carry the game. They just work damn well.