Like a lot of people I began to like the Simpsons less and less. I don't remember exactly where I started to notice I wasn't enjoying it as much, but it was around Season 21 where I dropped off completely. It just wasn't as funny to me anymore, and it didn't help that new episodes were been aired randomly (meaning they expected people to have a vested interest in the showm which I did not).
Thanks to the internet, I have been able to go back and watch the series I have missed (side note: I do own all of the currently released seasons on DVD (aside from the early released Season 20 because I hope they do a better/fuller version to make up for the crummy version they already released), and I do plan to get any future seasons that are released), and while I am enjoying it again it still doesn't feel the same.
I wholeheartedly agree with what Bob said back in one of the junk drawer episodes: the simpsons is still funny, but not "laugh out loud, rewatch until you've completely memorized the entire episode" funny.
The problem, at least for me, is now the Simpsons are making references targetted at my, or close to my, generation. In the past, the show would parody some movie or event I was too young to really know about and that actually intrigued me more. I didn't know that Burn's speech when he runs for Governer was a reference to Citizen Kane or the statue head in Bart's bed was a reference to Godfather. It made it appear, at least to me, that the Simpsons was funny on its own merit.
However, now that the Simpsons is targetting things I'm aware of, I'm noticing the references more and more. It made what I used to believe was a show that could stand on its own merits into a show that was just filled with cliched references.
And because apparently you can't talk about the Simpsons without one:
5. Trash of the Titans
4. Bart Stops the Smell the Roosevelts
3. Homer's Enemy
2. Radioactive Man
1. Lisa the Simpson
With honorable mention going to "The Frying Game" because Homer taking the blame for the crime so that Marge would get to live is, in my opinion, one of the sweetest things anyone has done ever.