In my opinion, here are my champs that are pretty OP and almost guarantee a win (unless your teammates are feeding or the enemy team is turtling and knocking you into their fountain)
Lee Sin - is really OP since he can dash to minions/teammates if he's in trouble, or dash to minions to position for his really huge ranged Q, which also is a gap closer. His ulti is also good for positioning the enemy for death and for 1 v 1's. Any champ with high mobility is good since the map is so small and the fact that he can Q across the thick wall between mid jungle and top quite easily to initiate top ganks.
Shen - he's good since he's unkillable if you don't play stupidly. He has tanky base stats (especially magic resist), can dash through walls with his taunt, and can ulti to a teammate if necessary to escape. His taunt can also cross the thick wall in the middle - top to surprise gank or gank people hiding in the two bushes on top. His taunt is also incredible in TT since he will usually be able to get all 3 team members (as opposed to 5s where it's rare to get the whole team in one taunt) which will disrupt everything, and his ulti will prevent most ganks.
Brand - He's good since there's only 3 enemies. This means more bounces for his ult, which can do massive damage per bounce (around 300 damage, and he has 5 bounces so 1500 damage at the very least), and his single-target stun will take out 1/3 of the team as opposed to 1/5 of the team in SR. Additionally, his AOE spell will most likely be able to get all the enemy team since there's only 3 enemies.
People like Riven and Shaco and stuff are pretty OP too, but it's not necessarily a guaranteed win for you if you play them since a lot of it depends on your skill and positioning.