Remember not what you think is the most powerful, but the most fun to play.
Oh yeah!
Don't forget to post why!
I like the warlord because what can be more fun than not letting monsters having the fun of killing you?
You can just see their shock when a giant invisible yellow shield stops them from killing you.
My top 5 are:
1. Warlord
2. Paladin
3.Rogue
4.Thief
5.Bloodmage