Well ain't that the million dollar question. There are so many douches in Skyrim I think Alduin has the right idea
But if I had to chose my top 5 would be, in no particular order:
Maven Black Briar, the condescending old Hagraven is the only problem I have with signing up with the Legion.
Ancano, he and Maven would make a great couple methinks.
Mercer Frey, as if being a double crosser wasn't enough he's lousy at what he does and has the nerve to tell you how to do things. Don't set off any traps he says as he sets one off
Braith, one of the several kids who make me wish for an XBox mod to kill children.
And last but not least, Delphine. Ugh, I just can't.