Pepe Le Pew
Foghorn Leghorn (Some of the funniest putdowns ever recorded)
Rocky and Bulwinkle (The Counterfeit Box Top episode was pulled from its original air date; the Cereal sponsors objected to the idea of copying cereal box tops)
Animaniacs (I think a wonderful re-creation and homage to Warner classic cartoons)
Courage The Cowardly Dog
Pinky and The Brain
Power Puff Girls (Their Beatles episode was clever; most of the lines spoken are words from Beatles songs)
Dexter's Laboratory (One of the best ideas is what a dog has to say when Dexter is able give it the power of speech)
South Park (Merciless with celebrities and religions)
Can't choose just one, but in no particular order:
Ren and Stimpy (I own the entire collection)
Beavis and Butthead (also own the full set)
Star Trek The Animated Series
Any of the Gundam series
And the list goes on and on and on...
The greatest cartoon of all time, is, without a doubt, by a 1973 Belmont Stakes margin, The Rabbit of Seville, starring Bugs Bunny and Elmer Fudd. I wanted to post a link, but YouTube charges to watch this classic. It was available on DailyMotion, but my work blocks access, so for all I know, it might be only available for a fee.
Everyone who didn't have The Rabbit of Seville as their answer is wrong.