So far I haven't really gone that far into any of the stories but I've seen most of them through the level 1-10 stage.
Jedi Knight: I think the best part of playing this story is Kira. Choosing that one option when you leave Coruscant to Taris "Pick something" and her response is amazing. "Re-program the ship to call me master. Done." I died laughing to that.
Trooper: Kinda took the storyline as a good trooper trying to end the corruption within the ranks. Rather boring.
Sith Warrior: I like how you get to kill so many people on your path to power. Vette also provides a good amount of comical relief like Kira did for the Knight. Plan on making this character completely evil.
Sith Inquisitor: I didn't exactly play this one I actually got to see most of the cut scenes since I'm playing with someone else and he chose Inquisitor. Rather shocking seeing how different that side is. Most likely will make one later.
Bounty Hunter: So far this one is really fun to play through, yet also the most conflicting on whether to be light or dark. I thought I was going to be able to go through and pick all the dark side choices I wanted. Mako doesn't agree. So I'm most likely going to turn Light Side, which for the Empire still isn't really that light.
Imperial Agent: Again only seeing cut scenes through my friend playing the class, but its very James Bond which looks like it could be fun.
My vote is for Bounty Hunter.