The Ren & Stimpy Show (1991)
Doug (1991)
Rugrats (1991)
Rocko's Modern Life (1993)
Aaahh!!! Real Monsters (1994)
KaBlam! (1996)
Hey Arnold! (1996)
The Angry Beavers (1997)
CatDog (1998)
The Wild Thornberrys (1998)
Rocket Power (1999)
SpongeBob Square Pants (1999)
Pre-Movie Spongebob was by far the best. The jokes were clever and funny. And there were a ton of iconic moments.
Post-Movie Spongebob is trash though.
The second best would probably be The Rugrats. Because it had a really long run and is also iconic.
My other favorites are Hey Arnold, Rocko, Thornberry's, Catdog, and Rocket Power.
I've also been watching a lot of Angry Beavers lately on The Splat, and realize it's a very underrated show.