S1 - Some Enchanted Evening. Just to be a dick I'm picking the original version, some of which is leaked on YT.
S2 - Blood Feud. The drama from Mr. Burns dying, Homer trying to retrieve the letter he wrote, Burns plotting evil against Homer after reading the letter... damn this is good stuff.
S3 - Flaming Moes. This is over-the-top surreal (do people really think the Simpsons didn't get that way until later seasons?) but there's so much comedy from Homer being a bitter crank when he's in the right this has to be the winner.
S4 - Marge vs. the Monorail. "Batman's a scientist" "Yeah, well, my plane leaves in less than a minute" "I call the big one Bitey"
S5 - Homer in Space or maybe Homer Goes to College
S6 - probably A Star is Burns because that's the episode I keep mentally rewatching. The Jay Sherman x-over provided some good jokes without being obtrusive.
S7 - Two Bad Neighbors. Normally I don't care about roasting political figures but the personality they gave Bush and his interactions with Bart and Homer totally sold it.
S8 - Itchy & Scratchy & Poochie Show. Whatever.
At this point we're getting into episodes I only saw once or never at all. I've got like 22 years worth of Simpsons to catch up on.
Honorable mentions: Who Shot Mr. Burns had some riveting parts, although the solution was kind of a cop-out. The Itchy & Scratchy-centered episodes because mini-cartoons are fun.