I haven't done much levelling but here is my opinion;
1) Imperial Agent (Level 50) - Amazing storyline. Act I was great, Act II was slow to start but amazing throughout and the end... Just amazing.
2) Sith Warrior (Level 32) - A fairly good storyline. Still haven't finished Act I but I can guess what happens... Which is the problem with it. You know what's going to happen and how it's going to end.
3) Jedi Knight (Level 24) - Fantastic so far. I'm not usually a 'light-side' guy, but it is really good. I can kind of predict what will happen but there's a part of me that has no idea.
4) Bounty Hunter (Level 26) - Good, solid storyline. Haven't got very far through Act I so I haven't really seen the best of it. I can see it being a very good story if I can commit
5) Trooper (Level 14) - Ugh. Getting off of Ord Mantell was a chore. The end of it was quite good, but the rest was boring. I can't see myself levelling this until very late on. I want to though, I can see it having great potential.
6) Sith Inquisitor (Level 14) - Korriban was fun. But I can see myself becoming an errand boy throughout most of Act I.
That's all I've tried. I need to stick to one character but I just can't! Most of them have good stories I just want to do them all.