I'll say SpongeBob for this one. While it only truly shined for less than half the amount of time it's been on the air, everything from 1999-2003 (including the movie) was just so brilliantly funny that it beats everything else I've seen and almost makes up for all of the crap made since. Almost.
Granted, I actually have yet to see Invader Zim, so my choice could change if that is as brilliant as everyone says...
As for some runner ups, I'd choose Chowder, Ed Edd n' Eddy, and Avatar: The Last Airbender.