My favorites are:
Nickelodeon:
Spongebob
Ren and Stimpy
Angry Beavers
Rocko's Modern Life
Invader Zim
Cartoon Network:
Ed, Edd n Eddy
Dexter's Laboratory
Courage the Cowardly Dog
Samurai Jack
Disney:
Disney's House of Mouse
Recess
DuckTales
Gargoyles