Fine, I'm game, I'll reply.
My answer would be a tie between two one shot characters.
First is "Never Leave Steve" (First seen or not rather seen in 2004's Squeeze the Day) Hey what can I say, I like a good play on words plus there is also the enigma factor Who was he? What did he look like, why did he never leave his room..save for that one time?
Sometime the best comedy is often left to the viewers imaginations.
The other would be Morsery (From 2009's Read em and Weap)
Only because I am part of the small percentage of viewers that actually got the reference behind the character.
http://en.wikipedia.org/wiki/Morrissey