In a game with as many variations as CF, it's difficult to say. With affinities, legacies, specs, shapefoci, etc., it's possible to have success with any number of classes.

I've played almost every class to at least the mid-ranks. I tend to enjoy classes that are self-reliant and stay away from those that are mob-related (conjies, necros, druids, etc.)

At the moment, favorites are probably:

Invoker
Warrior
Thief
Ranger

Niv