I have one toon of each class. No way I can pick a favorite. LOL Sure, two toons are significantly higher lvl than the other two (and there are about 3 levels separating those two highest lvl toons), but I like each one of my toons for different reasons.
(I'm assuming that you can only vote for ONE?? Cause I definitely DO have strong preferences for a certain subclass in each category.) LOL
My favorites, in order, are Fire Wizard and Shadow Priest.
Ice Wizard and Holy Priest are my runner-ups.
I have characters in every class and enjoy them all but the Warrior classes are my least played.
All of that is based more on my personal playstyle and is not meant to be a comment on which classes I think are "best" by any measure. I tend to most enjoy magic users and least enjoy melee characters in every MMO and fantasy game I have ever played. When people ask me what is the "best" character, my response is that whichever most appeal to you is generally the best for you to play.