This list of insults found via Digg makes me wonder what the formula is for creating a great insult. There definitely seems to be a typology.

Among the most popular is the backhanded complement:

He is a self-made man who worships his creator. — John Bright
His mind is so open that the wind whistles through it. — Heywood Braun
I can’t believe that out of 100,000 sperm, you were the quickest. — Steven Pearl

There’s the insinuating question:

When you go to the mind reader, do you get half price? — David Letterman
Why are we honoring this man? Have we run out of human beings? — Milton Berle
I want to reach your mind - where is it currently located? — Ashleigh Brilliant

There are simple metaphors:

He has the attention span of a lightning bolt. — Robert Redford
He has no more backbone than a chocolate éclair. — Theodore Roosevelt
The best part of you ran down your mother’s legs. — Jackie Gleason

There’s the complement with a clarification:

He writes his plays for the ages–the ages between five and twelve. — George Nathan (about George Bernard Shaw)
He was happily married - but his wife wasn’t. — Victor Borge
No one can have a higher opinion of him than I have; and I think he’s a dirty little beast. — W. S. Gilbert

And then there seems to be a long tradition of openly wishing death upon people. I’d call these curses more than insults:

He is one of those people who would be enormously improved by death. — H. H. Munro
I didn’t attend the funeral, but I sent a nice letter saying I approved of it. — Mark Twain
There’s nothing wrong with you that reincarnation won’t cure. — Jack E. Leonard