Miss Fortune is super aggro and can push lane against anyone. If I get her on blue team with a decent support, that tower is history. Also, people don't realize how useful it is to use her ult to clear and entire wave of mins so yours can get to the tower faster and take it out. Her CD on it is low enough to justify it.
I used to love Teemo, don't play him as much anymore because if youhave a bad game as Teemo everyone on your team will hate you for it. His kit is great for sniping and split pushing, but everyone else wants me to play him AP (which is also good, but not my style as much).
Nasus is awesome top, but a lot of people can shut him down. Give him a good partner and he can hold any position, though.
I just bought both Amumu and KAyle. They're just as god as everyone acts like they are. Amumu is just so tanky right out he gate, you can gank into any lane as lon as you aren't really outnumbered and be sure you'll at least survive if not secure the kill for the lane. Kayle's abilities are such that you can use them in pretty much any situation (as opposed to say, Teemo, who just has to sit out teamfights). She'll never run out of mana because all her stuff is so mana cheap and her E that turns her ranged is so cheap and has such good CD she's basically a ranged out of the box. It's kinda funny, the very best LOL champs are either 6300 IP or 450 IP...(Ashe, Garen, Kayle)