Considering I have just about one of everything, that's not a very easy answer. I'll take a few moments to highlight what I like or why I chose to play whatever I played.
Sith Juggernaut - played to 50 - Amazing Story! I was absolutely ensnared by the story line and I really enjoyed the play style. You have so much control on the battlefield with tons of CC. Either running tank spec or DPS i had so much fun smashing peoples face in or harassing cast bars like it was nobody's business.
Smuggler - First toon(s), played to 29 (gunslinger & scoundrel) - Because I needed a wookie. I have always loved wookies and was kinda disappointed when you couldn't play one.
Sage - still playing @ 32 - The caster play style. I love the mobility of the class especially with the DPS build I've come to love. I love the fact that I have just enough burst and utility to get the job done against almost anyone in a 1v1 and I also love the fact that I can pull 250k+ dmg at my current level. The only gripe I have with the class is the story line. Besides the tale end of chapter one I have been banging my head against the keyboard out of boredom.