Today is the tenth day of the marathon. I can't pinpoint when the Simpsons went downhill; I've never figured out my favorite season.
Yesterday I caught one of my favorite episodes: Season 18's 24 Minutes, a parody of 24 and Springfield's Counter Truancy Unit. Lisa and her team save the school bake sale from a stink bomb.
I also set the DVR to record The Book Job from Season 23. I had saved that on our old DVR.
Other favorite episodes:
Moaning Lisa and Life on the Fast Lane from Season 1
Season 3's Homer at the Bat and Black Widower
A Streetcar Named Marge, I Love Lisa and Last Exit to Springfield from Season 4
Cape Feare from Season 5
Sideshow Bob Roberts and Homer Badman from Season 6
Radioactive Man, A Fish Called Selma, Bart on the Road and Summer of 4 Ft. 2 from Season 7
A Millhouse Divided and Brother from Another Series from Season 8
Season 11's Take My Wife Sleaze with the romantic line, "My wife is not a doobie, to be passed around from person to person! I made a sacred vow, on my wedding day, to bogart her for the rest of my life." (Thanks IMDB.)
Trilogy of Error, New Kids on the Blecch and Skinners Sense of Snow (I especially love the title of the latter) from Season 12
She of Little Faith from Season 13
Season 18's 24 Minutes
The Book Job from Season 23
I made this list looking at the FXX marathon schedule as well as The Simpsons wiki. I may have missed a few and may change my mind depending on my mood. I didn't list any Treehouse of Horror episodes even though I could list some individual segments as favorites. "When are they going to get to the fireworks factory?" is one of our favorite quotes; it's from The Itchy and Scratchy and Poochie Show from Season 8. I love many of the Itchy and Scratchy cartoons and all appearances of Troy McClure.
I guess things went downhill from Season 12--that's the last season I had more than one favorite. Trilogy of Error is my all-time favorite episode. I've listed 23 here. Maybe someday I'll select my top 10.