My current favourite is mage: He can take out entire groups of enemies, but also requires a lot of skill. Good use of mage makes him have full mana by the time your old ball is depleted.
I love the mage. The fact that you can control his magic makes him a class with much depth and potential. Very fun ability to wield.
I also enjoy the Necromancer as I love when you control hordes of things. The Adventurer is awesome because of speed, and I love the Barbarian's giant sword. It's feels awesome to use.
The only class I dislike is the priest. Without others to share his magic on, he's pretty much just a guy with a lot of HP. Maybe his magic could also turn undead?