Maul kicked ass, but had no lines (ok, he had all of 1 line in the entire move). He was more style than substance really.
Jango was a punk that got wasted by Mace Windu.
Grievous was a silly wheezing droid that never came off as menacing.
Jabba is in more than one movie, so he doesn't count — even if you want to disregard Episode IV SE, he does make a cameo appearance in TPM.
Tarkin, hands down.
He's a cold, ruthless, classical sort of villain. His iconic scene of villainy is the scene where he orders the destruction of Alderaan to get Leia to talk, and when she finally does talk, he turns around and orders Alderaan's destruction anyway. He has the best dialogue of all the villains. He also is the only character besides Palpatine himself that manages to reign in Vader, which bumps his impressibility up a few notches. He also has the blind arrogance of a great villain: "Evacuate? In our moment of triumph? I think you overestimate their chances."