I found that the sith inquisitor has the best storyline and is a lot of fun to play
There are so many differing opinions here that its a good sign that the classes are pretty balanced and all of them have something for somebody.
For me personally, having played 4 classes to high level, I say sorcerer has the absolute worst story. Warrior has best followed by bounty hunter.
My hybrid dps sorcerer is however, when properly pve geared, my most capable and fun to play character in pve, despite having relatively short attack rotation. My marauder is also great fun and really complex to play well, particularly in pvp. I agree with beforementioned powertechs aesthetic appeal.