I think that The Sipmsons, Family Guy, and Spomgebob, all share the same fate. They started off so strongly, that they can no longer top themselves.
I've totally given up on Spongebob, but I still watch the other 2 because they are still funny, just not instant classics like their older episodes. I still catch Spongebob sometimes but I only watch if its episode I've already seen. Fairly Odd Parents was alright but I got tired of it. I also used to watch Blues Clues, but I don't like the new guy. Long Live Steve!
I thought I would list some old cartoons I used to watch starting with one of my all time favorites, but in no particular order after that.
Centurions
G.I Joe
Captain Planet
Johnny Quest
Batman (every single kind ever, including the live action show with spandex Robin *POW* lol)
X-Men
Spiderman (same as Batman, including the short lived live action show)
Transformers(all kinds)
Masters of the Universe
Super Friends
Justice League (all kinds)
Thundercats
King Arthur( I can't remember if that was the name, but that's what it was about)
Smurfs
Snorks
Care Bears
Teenage Mutant Ninja Turtles
(I also forgot to list Aqua Teen Hunger Force in my first post)
Then there are two that I loved that I can't remember the names of;
1. It was set in space and humans were fighting against these alien creatures and against space pirates. They used these bad-ass exo-skeletons to fight. I remember in the last season I saw they found this alien like lab or something where the aliens were being bred. I remember the last episode I saw they teamed up with the Space Pirates to try and take out the aliens for good. Then the captain gave a little monologue at the end saying, "maybe someday there will be an enemy so strong, we'll have to join forces with the aliens" not an exact quote but we're talking like 12 years ago, but that stuck with me because I always wondered about what kind of enemy would come next.
2. The other show was on Cartoon Network way back in the day but it was more popular to my knowledge. They even made an appearance on Family Guy. It was with pre-historic warriors, they had a pterodactyl, and triceratop for allies, along with these two blob like creatures. It was a man, his (hot if I remember correctly) wife, and their son.
edit: mistakes fixed