Story: Agent > Warrior > Hunter > Inquisitor
Gameplay: Hunter (Biased) > Inquisitor > Warrior > Agent.
Hunter (Powertech) is by far my favorite, but really they're all good. If I rated each AC by "Fun":
Powertech: 10
Juggernaut: 8
Sorcerer: 7.5
Assassin: 7.5
Sniper: 7
Marauder: 7
Operative: 6.5
Mercenary: 5
Didn't like the Mercenary much (preferred Commando), felt Operative was weak, rest were fun. I have a 55 of every AC, and would recommend Sniper or Sorcerer for ranged damage, Juggernaut or Assassin for Tanking, and Powertech or Marauder for Melee damage. Of course, if you plan on doing high level PvE (Raiding), then reverse this list, as Merc and Operative have the top DPS.