Spongebob Squarepants
Avatar: The Last Airbender
Rugrats
The Ren & Stimpy Show
Spongebob and Avatar are two of the most obvious picks for a rushmore out of any rushmore we've ever done. Rugrats is there as the Klasky-Csupo rep and the third most reliable cash cow after Spongebob and Avatar (not to mention it was basically the mascot franchise before Spongebob really took off).
There are lots of shows that have arguments for the last slot based on things like longevity, influence on media and long-lasting popularity, but Ren & Stimpy being the show that more or less put extremely cartoony, original, storyboard- and creator-driven cartoons on the map is massive. In spite of the fact there's only around 20 episodes of it worth watching. Without it not only do most Nicktoons after it not exist most cartoons period after it don't exist. Most cartoons with as many controversies behind it as it has had would have long stop being mentioned if they were not also that impotant, because Ren & Stimpy's importance to both the Nicktoons and to animation is very difficult to overstate.
HMs: Rocko's Modern Life, Hey Arnold!, The Fairly OddParents, Invader Zim, Doug, The Loud House
Born to lose, live to win!