The bit in 'Lemon Of Troy' when the Springfielders are about to leave Shelbyville with the Lemon Tree on top of Flanders' RV, but the tree hits the 'Now Leaving Shelbyville' sign and falls off. I crack up every time at that, great episode. Couldn't find a video of that moment so here's the bit that immediately preceeds it:
http://www.youtube.com/watch?v=GrCYX5lxVQo&feature=related
Rough Sausage said:
The bit where Homer misses church, and lounges around in his home in his dressing gown and what not. He sets his house on fire, and Ned throws a mattress out of the top window, and then Homer onto the mattress. Homer bounces off the mattress, into the ground floor window. Absolutely best moment ever. Couldn't find a video for it, unfortunately.
And a great moment just after that, when Flanders jumps out to get him, and does a perfect somersault off the mattress and through the window to rescue Homer again xD.
One more moment, and that's Lisa's tearful goodbye of her substitute teacher, Mr. Bergstom. That's the most heartwarming moment in Simpsons history, prehaps all of Animation history.