My favorite is Grand Moff Wilhuff Tarkin...
Tarkin seems, to me anyway, to have a certain level of authority above Darth Vader, and he's cool and calculating... Something about him echoes that his ONLY flaw was not giving the rebels more credit than they were due. His assumption the jedi fire was put out was his ultimate downfall... But honestly, could anyone have expected Luke to be there? Doesn't seem likely to me either.
Peter Cushing is a personal fav of mine in other movies though as well, so I'm a bit biased in that I just enjoy him as an actor. I consider his the best performance of Star Wars overall.
The 2nd Imperial I liked was General Maximillian Veers... He's the epitomy of a military hard@$$ I think. He's the General Irwin Rommel of his era and place.
He's regal, efficient, obedient, confident... He's also willing to throw on the armor and get in the trenches with his troops. He's no pansy who makes his subordinates lead an important charge. He takes the reigns and dictates the battlefield personally.
Some officers work from the BCC, Veers works from the battlefield itself.
He's also confident around Vader... No quiver in his voice, no fear... He knows he's good at his job, and won't be choked... Or so he thinks anyway. So did Captain Needa I guess, who did seem confident himself.
Plus, if you follow EU completely (Or just like this part), Veers is one of the few to escape the Death Star to become the prominent officers within the Galactic Empire.
Depending what POV you have, he could still be living beyond Endor, and he could still be the hell on 4 legs (Via the AT-AT, hehe) we saw on Hoth... Raining death from above on rebel forces for years past the Emperor's demise.
That's how I like to think of him anyway.