Me it's when Homer's looking at the picture on the stairway wall, and Bart's shown holding a paper cloud/bubble/comic-style-text making Homer say "I'm stupid", and then Homer says "Hey, I don't remember saying that!". Man that's the best. Any others? How about the 'I'll donate 1 million to charity....when pigs fly." Then Homer's BBQ pig flies by, and Smithers says "should I make out that check now sir?" and Mr. Burns says "no, I'd still rather not". Or the See my vest tune.