That of his villianous character Dr. Decay, who definately ain't Silver's buddy, nor pal, nor friend, not even a fan.
Here's a thing to bare in mind folks, a villian don't necessarilly have to be the most awsome looking or darkest looking character design, but can be goofy and very creative like this dude.
Heck Eggman's design alone is a hilarious one, a fat egg-shaped human with a bushy long mustache and clown like smile, yet he's one of the most nastiest folks around.