The Shadow/Assassin I only played a little bit but their tiny energy bar and huge costs is a pain. They can use like two abilities before running out of energy.
Sorcerer is probably second place, they have a huge energy bar that never runs out in normal fights, and they throw all sorts of lightning and cool looking "dark" effects around. Not as impressive as the BH's explosions, but much more exciting than the Sage's pebbles. Speaking of which... "I'm going to throw pebbles and dust at you until you die" was not what I imagined the class would be like.
Lastly, the Smuggler/Agent is so weird and diverse among specs it's hard to pin down. I put a lot of time into Operative, and it's just a really messed up class, the healing tree is nice but the whole vibroknife system is, well, bad/unfinished. The Sniper/Gunslinger is a lot more fun, but without stealth it feels like it's missing something IMO. What kind of agent/sniper can't stealth?
Assassin actually is a lot of fun once you learn how to manage your energy. You can't just spam everything mindlessly or you will run out fast. But there's a talent in the Deception tree that increases your energy regen in stealth and for the first 6 seconds after you leave stealth. So that helps a lot provided you're in stealth as much as possible. If you open a fight out of stealth, you can get out some initial burst while maintaining good energy regen. After that it's just a matter of weaving in your free strike sometimes so you don't get low and being smart about what skills you use when. Fun once you get the hang of it.
I've only tried Sniper in the teens and its fun so far, but I've played an Operative healer into the 20s and that class is fun. It's definitely very unique but even in the 20s I feel like something is missing. It's a late bloomer I think sadly.
DPS sage is my main and that's a lot of fun. I actually prefer the boulders, pebbles, and pure force attacks to the Sorceror's lightning - having all your attacks be lightning gets old fast, as does the storyline.
Only just started making Trooper, BH, and Knight alts so not much to say there but I do find them all interesting. All in all I'd have to say Sage is the best. None are really bad, though - I'm having fun with all of them! Might be worth noting, too that I'm not a huge Star Wars fan though so I had no franchise expectations that the game had to live up to lol.
Edit: I have to agree with the previous poster who didn't like Hutta. It just sucks so bad. And to add to his comments, it's also too easy to find yourself at level 4 or 5 and all of your quests are orange, yet you have no more to do. Both of the Republic starting worlds are sooo much better. Korriban is pretty good, too.