The Simpsons started going downhill ten years ago, right around the time Conan O'Brien quit as one of the writers. I'm a King of the Hill fan now.
Some of the absolute best Simpson shows were written by Phil Hartman (the Monorail one and the one with the musical version of Planet of the Apes are my favorite). Without Lionel Hutz and Troy McClure the Simpsons have just been missing a large talent.