It's funny how everybody went the route of looks.
When it comes to that, the least flattering I've ever gotten was probably "Dude, you look like that guy from A Beautiful Mind." Was it because I look like Russell Crowe or because I somehow look schizophrenic? I don't think I look a thing like Russell Crowe.
Personality wise, a consistent and rather unflattering comparison I get is to the character Q from Star Trek TNG, DS9, and Voyager. I can think of a few good qualities of his, but he's mostly an antagonistic jerk.
Come to think of it, I also get House a lot. It's a wonder I have friends and find clients.