I personally cannot chose between Just one. though Fozzie would be it. Actaully, I like them each on different levels.
Fozzie's funny
Dr.Teeth is Groovey (and designed really well)
Kermit is the all around nice guy who sometimes loses his cool
Cookie Monster shows the Barbaric instinct that we all deep down share (sometimes, when no one's around, I'll eat a cookie like this guy)
I can't get to specific or I'll be here all day!