Tom & Jerry, hands down. Shortly followed by Loony Toons. Anything with silly
slapstick humor, really >.>
SpongeBob is awesome too, but his status as a classic can be argued... But
you just can't argue with a sentient sponge who has a manic laughter. Srsly.

I'd say Tom & Jerry, Pop Eye, and pretty much all the old Looney Toon related shows (Like Bugs Bunny and Daffy Duck and whatnot.)

...those "rocko's modern life" and others are good cartoons but 90-s cartoons are not classics (for me at least).
Ghostbusters(This song reminds me my childhood!)

^ So that's where Juntalis' last avatar came from.

Oh: Beetlejuice and Bobby's World.
EDIT: Especially Beetlejuice. I mean, who doesn't love BJ?

I used to love Hey Arnold!

but, that was the good old days.

Invader Zim FTW.

lemme see...
the first TMNT was good... so was Scooby Doo and the wretched wannabes that followed.

(captain caveman and the teenangels come to mind... god, what a waste)

But for cartoon network, theres a special place in my heart for Cow and Chicken, and all of David Fleiss' toons. Ren and Stimpy is a close second (even those episodes not made by John K.)