6. Bart's Earned A Day Off (Season 23, Episode 14 - At Long Last Leave)
One of the all-time best chalkboard gags sees Bart taking a back-seat while his buddy Milhouse takes over, declaring that Bart's earned a break after spending 499 episodes writing on that damn chalkboard, for this is the 500th episode, an impressive if ultimately arbitrary milestone (as a previous chalkboard gag made clear). And of course, it's practically impossible to watch this gag without imagining that Bart somehow duped Milhouse into doing the punishment for him, rather than his buddy simply doing it out of the goodness of his own heart.