Just for fun, I looked at Elementz' solo queue tier list (Yes, I know, lol Elementz) just to see what he thinks on the top AP champs.
In order, they are: Morgana, Ryze, Cass, Karthus, Kennen, Kass, and Xerath in tier one. Ahri's the top AP in tier 2.
I feel bad for poor Heimer, alllll the way down at the bottom. He was one of the first champs I learned to play really well.