Can't make the fairest of judgments, as I've got 10 Imperials and only 2 'Pubs (JK & JC)
Playing through all Imperial classes 2 times+, I can pretty safely judge them:
Most influential: Sith Inquisitor, by far.
Most powerful: Sith Warrior.
Both JK and JC don't match up. The Jedi Knight is just annoying, and so badly written. Almost like a deus-ex-machina for the Republic. They are only class that actually does anything, and what they do is just... meh. Almost like Luke post-films, they're just a tool for lazy writing on Republic side. They couldn't think of a good way to make the Republic win, so they just bring in the Knight to 'win' in every sense.