Futurama before its episodic movies is probably my favorite show of all time. I'm kind of iffy on the Comedy Central episodes. Last week's episode was really bad, but not all of them are.
I've seen almost every episode of King of the Hill more than twice thanks to it running two episodes twice on Adult Swim just about every night. Yet, I still find all the real world problems the characters run into interesting and sometimes hilarious. It's kind of a breath of fresh air compared to all the zany stuff you would run into with other cartoons. It's very hard to find yourself hating the conservative Hank Hill, even if you disagree with him.
Spongebob was great, but the movie and everything after is stale. I don't really watch many shows attentively anymore, so all I can say about newer cartoons like Adventure Time is that I wish the dog character was drawn a bit differently. Regular Show seems great.
I started watching Avatar kind of halfway through the series, but it was definitely good, I just realized it late.
Growing up, I probably watched Nickolodeon more than Cartoon Network, so it's always nice to look back and think about Rugrats, Doug, Hey Arnold, Rocko's Modern Life, and Angry Beavers. That doesn't mean I didn't flip the channel to watch Dexter's Lab and Cow & Chicken, among other shows.
Also, I found myself going to this
website a lot my last semester in college, since I didn't have a TV. It streams a lot of 90's and Adult Swim stuff.