The Simpsons, like Seinfeld, is another show you could fill an entire list with and there would be no wrong answers.
Besides Marge vs. The Monorail, you could easily put "You Only Move Twice" (the Hank Scorpio episode), Treehouse of Horror III and V, the Stonecutters episode, "22 Short Films About Springfield" and at least a dozen others.
Among the animated shows, though, I think my vote would go to Futurama's "Parasites Lost." Fry gets worms in his colon and the crew has to shrink down to microscopic size and go inside him to get them out, in an homage to "Fantastic Voyage."
Every little zinger and throwaway line ("Gumbercules? I LOVE that guy!") and small sight gag (Fry getting in an elevator button inside his body and the buttons read brain, lungs and ball room) connect, and they're fired out non-stop. The jokes have some wit to them, and the entire story actually advances the plot of the series a bit. It's a darn masterpiece.
Another great one is the Season 1 Venture Bros. episode "Tag Sale! You're It" The amount of world building they do in 22 minutes is staggering. They introduce several great background characters and long-running plot lines, and have about eight different subplots boiling at once, yet somehow tie everything together at the end. You can watch that one episode without having ever seen another and have a great time, but as you do watch more of the series you realize how much they established in that one episode. You could teach a clash in story plotting from that one.