In terms of pure explosive firepower, I would say:
Adult Annie
Anivia
Veigar
Ryze
Karthus
Nunu
Starchild Soraka (before she turned WW into werewolf)
Lux
Janna (It's a tornado!)
Ezreal

In terms of fighting ability:

Jax
Garen
Katarina
Yi
Akali
Eve
Irelia
Xin Zhao
Tryndamere
Sivir

NOT NECESSARILY IN THAT ORDER

Annie is regarded as the most powerfull mage in lore, atleast when she reaches adult hood.

Im surprised nobody mentioned ryze, he pretty knows everything about arcane magic.

I'm surprised Maokai is being so neglected. Not only did he single-handedly defeat all six champions participating in a match (who they were exactly doesn't much matter considering that he beat all of them), but he's essentially magic incarnate. The only other champion with that kind of assumed power is Annie.

The thread was made before Maokai. Just updated it. =)

My rankings:
1.Kassadin
2.Malzahar
3.Anivia
4.Jax
5.Mordekaiser
6.Vladimir
7.rammus
8.Kayle
9.Soraka
10.Morgana

And of course, there is the issue of the difference in classe : demon, demi-god, human, etc.
9.V

Not to be anti Maokai, but they never said what lvl the summoners were at when he slew them. I mean, an uncontrolled treant w/ full power against 6 lvl 2 or 3 controlled summoners...

i cant believe noones mentioning xin zhao, he was slated to fight 300 gladiators at once... that means he must have defeated a very sizable force already,

I still thik Sona should be on the list, The ability to entrance anyone of any power is a usefull skill to say the most.

What would Malzahar and Kassadin classify as... I know they WERE human and are still mostly human, but would the classify as humanoid demons now or superhumans?