When Homer started licking toads and became a missionary.
When Homer ends an episode by turning to Marge and saying romantically
"Well there's one thing his (Mr Burns) money can't buy"
"What's that Homey?"
"...a dinosaur".
At Homer running on the treadmill with his huge stomach wibbling:
'It's like a lava lamp...!'
Or the one with the spoof of James Bond, where the Simpsons move and Homer goes to work for the big baddie, but he doesn't ever realise it, and Marge starts drinking during the day!
Gotta be the episode when Burns starts a casino and goes all germ mad. The minature plane he tells smithers to get into, the spruce moose. smithers protests at getting in a miniture plane and burns pulls out a gun, "i said get in!!"
Difficult to choose, there's so many great ones, amongst my favourites are the one with Shary Bobbins, the one where Flanders opens his left-handed shop, the one with the Radioactive Man film, the Stonecutters one, the one with Mark Hamill, Who Shot Mr Burns?, 22 Short Films About Springfield, Flaming Moe's, and loads more.
My fav is the Frank Grimes one - especially when the guy ends up killing himself because of Homer. I also liked the son of Frank Grimes � revenge episode. But as everyone says there are so many to choose from � I loved the way Flanders wife was killed too (because of Homer). Not mad on Lisa or Marge related episodes, prefer Bart and Homer (as usually more funny) :-)