The older episodes are the best, the recent ones (all released within about 3 years) I find absolutely horrible. I don't watch the simpsons when a new episode airs.
One of the greatest episodes would be the one where Homer works for a new company with an evil CEO bent on world domination. Mr. Scorpio!
Mr. Scorpio (talking over video line to U.N. Representatives): Just to show that I'm serious... watch this! (presses button and bridge explodes)
U.N representative #1: Good Lord! The 59th street bridge!
U.N. representative #2: Maybe it collapsed on its own.
U.N. representative #1: We can't possibly take that chance!
U.N. representative #2: You ALWAYS say that! This time I want to take a chance!
Another good one is when Marge becomes a cop,
Homer: Oh no! An illegal counterfeit jeans ring operating from my car hold!
The Stonecutters episode is a classic!
Marge: Homer, I want you to promise that you won't stalk Carl and Lennie.
Homer: (gestulating) I'm going out... to stalk.... Carl and Lennie... D'OH!
During Stoncutter initiation:
(Homer holding his right hand up in an oath to the sacred order of the Stonecutters)
Homer: And by the sacred parchment, I swear if I reveal the secrets of the Stonecutters may my stomach become bloated and my head be plucked of all but three hairs...
Moe: I think he should take a different oath!
Other random episode clips that come to my mind
Episode where Homer and Ned Flanders become good friends:
Homer chases after the Flanders' family Geo in a parody of Terminator 2
Maude: Go faster Neddy!
Ned: I can't! It's a GEO!!!
The episode where Apu loses the right to run the Kwik-E mart!
The monorail episode!
The episode where Homer becomes an astronaut
(Scientist dude addressing Homer and Barney on test evaluations)
Scientist: In a way you're both winners. But in a more precise manner, Barney is the winner!
(they toast with non-alcoholic wine, Barney becomes intoxicated and flys away on a jet pack)
Scientist: I don't understand, that was non-alcoholic wine! Okay then Homer, you're the winner by default.
Homer: Woo hoo! Dee Fault! The two sweetest words in the English language! Default! Default! (scientist incapacitates Homer with mini-club)
The Simpsons are at their best whenever they can parody or spoof classic/well-known movies, make jokes about politics, or portray serious life themes in a humorous manner. New episodes are missing all of those components. They are nowhere near as clever or thought out. I don't think Groening is all to blame, he's probably only focusing on his newer series: Futurama
But it's sad to see such a (once) well made show fall to its knees. It should have been let go with dignity, not drawn out into such a waste of 30 minutes of air time.
I think I've typed enough, although it may be surprising I have a friend who's even a greater Simpsons nut/fan(atic) than I. He probably knows everything there is to know about the show.