Ren and Stimpy and Rocko's Modern Life, because time has been so kind to those shows. Everything, from the writing to the voice acting to (in most cases) the animation has held up.
Spongebob (the first three seasons) and Invader Zim deserve honorable mention, for being so good at a time when most other Nicktoons were bad (or at least mediocre).
Rugrats (again, only the first three seasons), Doug and KaBlam! were often clever and funny, but I can't single out individual episodes for excellence like I can with the aforementioned Nicktoons.
Zim: "I thought the Tallest killed you."
Skoodge "Yeah, but I'm all right now."
Mad Jack the Pirate: "You were DEAD! How do you get better from being DEAD, you Scottish twit?!??"