OP, it depends on your play style. You wanna burn down a mob before it gets to you? Scout or mage isn't a bad option. You wanna be a CC person? Psi, mage and to a lesser extent summoner are possible. You'd rather heal the party? Healer and summoner have you covered. Tanking? Paladin for bosses and warriors for multiple mobs.
In Soviet Russia, MMO fails at you!