Rocky & Bullwinkle
Looney Toons
Ren & Stimpy
Batman: TAS
Iron Giant
Popeye
Simpsons
I got back into cartoons in the early 90's watching MTV's Liquid Television. Aeon Flux, Beavis & Butthead, The Head, Genie Junkie. All kinds of weird stuff.
Aqua Teen Hunger Force
Archer
Looney Toons/Merrie Melodies
The Simpsons, seasons 1-8
Thundarr the Barbarian
The New Adventures of Flash Gordon
Ren and Stimpy, season 1-2
Star Trek the Animated Series
Batman TAS
I think those are the ones I'd immediately stop and watch if I surfed by them on TV (or own the entire series of on those old solid media.)
Edit: Due credit for Rocky and Bullwinkle/Fractured Fairy Tales. Those were wonderfully subversive during the height of the Cold War.