I belive Invader ZIM was a very under-rated show. I actually enjoyed that hsow's humor, art style, and the theme: an alien trying to take over a world he doesn't understand and has no chance of doing so. It's very humorous, even the first episode. Most cartoons have a horrible first episode, mainly because it's the Pilot and meant to be experimental. The only other cartoon I can think of that had a rather good Pilot is Foster's Home for Imaginary Friends.
Jimmy Neutron is very creative, and not over-rated. However, as most cartoons, the newer episodes suck. And that really sucks.