Many great ones here. (Even though this subject has definitely been done before.) I have to choose the top ten (though I can't really rank them internally, they're too good), and that's still bound to miss out on some great ones.
"Life on the Fast Lane": I got close to crying at the end of this. A wonderful romance, plus bowling!
"Lisa's Substitute": Well, what everyone else said.
"Last Exit to Springfield": As Steve said once, almost every line and scene in this episode is quotable. From the initial McBain scene to "oh, I left the gas on"... splendid, pure fun.
"A Streetcar Named Marge": Another lovely look at the relationship of Marge and Homer, plus some of the best tunes ever.
"Treehouse of Horror V": Probably my favourite THoH, which is saying a lot. I'm lost for words on this one.
"Lemon of Troy": The episode that got me hooked on the show. Says it all, really.
"Who Shot Mr Burns?" (both parts): The episodes-plus-cliffhanger that ensured my *staying* hooked. Plus, it is a wondrous appearance by Mr Burns in all his evil glory.
"Treehouse of Horror VII": A close second. Lovely in all respects, especially the "Citizen Kang" segment.
"The Secret War of Lisa Simpson": Well... Lisa in the army. What more could anyone wish for?

Well... there are plenty more, but these were at the top of my mind. And you've got to admit, there's a lot to choose from.