I'm in the same boat. I've enjoyed all of my classes but the *surprise* was the Imperial Agent, Operative. I picked Operative because everyone else picked sniper, and I picked the darts DPS tree because everyone said it sucked.
It has actually been VERY fun and VERY different from all my other classes.
I'm looking to start up my 5th class. I haven't tried the following: Warrior, Bounty Hunger, Smuggler, Trooper. Any ideas?
Warrior and Trooper are my 2nd and 4th favorite classes. Trooper is really only rewarding if you play Light Side imo. Warrior can swing either Dark or Light, but I found it the most rewarding playing Light Side (almost Revan-esque).