Invader Zim has a unique art style, it's randomness adds to the humor, the way it portrays the human race and aliens as being more intelligent is funny, Gir is everyone's favorite, and the list goes on.
My favorite episode is the one when Zim's spaceship runs into a bee (YES, A FREAKING BEE) and crashes. Like, twice. The first time flying was to screw with the humans, the second time was recovering the broken ship from a group of alien-believers in a giant hollow pig-like ship.
Yes, so much random.