In terms of story, the Inquisitor's is absolutely fantastic, with a cast of amazing companions.
The Sith Warrior's story is also fun, but the companions I found dull. Vette's funny and easily the best one. You can shock her if you're evil which is also fun. However, if you are evil, Jaesa is literally insane. Almost to the point where it's creepy/annoying. Broonmark is dull. Malavai Quin...I won't to throw him out the airlock (especially later in the story). Pierce is okay.
The Assassin is great in PvP. Stealth, tanking, and even some great dps make them a challenge.
Juggernauts also do great dps and take a lot of hits, but lack stealth.
Personally, I prefer the Sith Assassin. But its up to you