There are so many awesome quotes I don't think I could pick just one... pretty much anything Harry Shearer does. I have laughed my ass off at so many Smithers/Burns exchanges, and even the most cynical people here have to admit that at least those exchanges have been of consistently high quality for the show's entire tenure.

On another note, I find it impossible to believe the earlier assertion that someone could only enjoy The Simpsons until 7 or 8 years old, because 90% of the best jokes on that show are subtle, ironic, intelligent, and you miss them if you blink; many of them include social and political subject matter that require some experience to put into context. There's no way an 8 year old kid is even going to catch most of those jokes, let alone comprehend them. I would be concerned whether someone who didn't find it interesting after eight years old is still not able to get those jokes today (not really an insult, since The Simpsons is the smartest show currently on television).