Sidekix

They screw the pooch by only having him at #3, but THIS LIST OF GREAT TV SIDEKICKS gets it right re: what made Seinfeld:

What made Seinfeld such a phenomenon? For as much attention as it received, it wasn’t Kramer’s insanity. Nor was it Elaine’s bitchy shallowness. It sure as hell wasn’t Jerry’s, uh, whatever the hell it was that Jerry did. No, it was George Costanza and his life. That was what was at the heart of the Seinfeld experience. Hell, it was such an integral part of the show’s success that it basically spawned a whole different show specifically about the George Costanza character, only this one was called Curb Your Enthusiasm and was about the real life George, Larry David. 

Leon from Curb coulda been on this list if they hadn't tragically underused him.  I'd also put Jackie from Roseanne and Carlton from The Fresh Prince on this list.  And hold up...no Ed Norton???!?!? Really?