Ahri here as well, reasons:
Mobility, true damage, multi-target that isn't reduced, taunt

ryze or fizz.. they may counter almost all champions..

Malzahar. He can push very easily with malefic vision and harass with it even get a few kills if someone gets too eager to attempt to kill him.

Ahri fan-cult nominates Ahri for bestest mid evar!

Seriously, though, she's not particularly hard-countered by many champions, her kit is a ton of fun to use, her escapes, chases, and ganks are legendary, and Charm gives good team utility. (Not to mention that it completely destroys certain champions - Master Yi in particular gets royally screwed by it, especially while trying to run away with his ultimate: "cannot be slowed" right back into your team, haha).

lb because she shuts down a lot of the "annoying" mids (no offense if you play them).

Also Zyra, but I find that she is weaker than a lot of the other mid champions after the nerfs.

Swain has been my favorite since i bought him in season 1.

Ryze, can't get enough of melting ppl with 450ip champ. Don't forget to turn on smartcast though.

Ryze- nuff said

Kassadin