There is a certain list of champs that can push their team to victory quite easily due to their resilient nature and damage during a large part of the game. They're not overpowered but playing them well will probably enhance your chances at winning (low ELO here by the way, so applies to 1000 - 1100).
Orianna - Easy poke, great CS with W, good pushing and sufficient escape potential (W + E shield + R magnet pull). As an Orianna main from about level 20 and now in ranked, I can say that she has few counters played right and, if doing OKAY, she does decent damage and DAT ULT. A little OP amount of damage, utility, range, and survivability. She's like a buff Mario-type character with a team-focused kit.
Leblanc - INSANE burst damage, great double escape, lots of utility (silence, snare, can do twice). As an assassin, she can easily take advantage of individual mistakes during the laning phase... AFTER bullying you down to half health. To me, she is viable as support, AP mid, and even a weird AD carry assassin that can easily snowball (TAKE MANA REGEN runes).
Diana - She is stable throughout the entire game, and as an assassin, she can punish mistakes really hard with her great burst damage. Her jungling is excellent due to this and is even on par with her laning. Late game, she is very much like a melee Leblanc.
Zyra - Like Orianna, she is an excellent wave clearer and her utility-damage is crucial to team fights. Excellent support as well--she ties with Leona and is right above Taric as my favorite laming support.
Twisted Fate - I have played him both AP and AD, and I honestly believe his AD carry/x2 Phantom Dancers is an instakill if you can lock an individual down late game. His utility is great, and every spell in his kit simply works. As an AP mid, he has great damage... if you can land his Q. Unfortunately, he falls off right at the end of the late game as pure AP when securing kills that again, his poke is insane).
Xerath - When not countered (i.e. Kassadin or Leblanc), his range and damage basically make him a reliable DPS source, and his poke is amazing, not to mention his wave clear. Even his W can be quickly remedied by quickly poking with his Q, and then using his ADDITIONAL MOVEMENT SPEED BOOST to escape. Not the best here (which is why he is lower).