the ethereal club
Best. Weapon. In. The. Game.
Let's see... from what I remember playing the classes (didn't play much as a Psion or an Animalist).
Gadgeteer: Seems to be the easiest class to play. A wide range of delevers (perhaps too many), bonus to damage, healing spell, and armor/weapon tweaks. Total powerhouse. Stun Gas is possibly the most powerful spell in the game, letting you adventure way above your head, especially with the occational stunning effect.
Elementalist: Just started a new one of these, and they're an interesting mix. Their spells are weak, but seem to scale with what they're fighting nicely. Lack of PP is the biggest problem. Still, spamming Lightning Bolt works rather nicely, as does Groundquake (Stun Gas seems better, though). Jesus is pretty spot on for early levels: fighter with a big honking "oh shit!" button. Fire shield is awesome.
Animalist: This is an interesting class. Lots of buffs, and their attack spells were pretty neat, too. Seem kinda like Turtle Tamers in that the spell can
miss. However, I seem to remember their spells joining with a normal attack, so an Ethereal Club will help pay for their attacks.
Psion: Er... really don't remember much, but I seem to recall having a hard time with these. The buffs were neat, but not too major, and the spells were neat, but didn't pack the punch of the Elementalist spells.