I know I said this before, but this is probably the pinnacle of The Simpsons. You Only Move Twice could be the best episode ever – Hank Scorpio is such a brilliant character, thanks in part to Albert Brooks' voice talent. Bart After Dark is more than the brilliant Grandpa .gif and We Put the Spring in Springfield song.
El Viaje Misterioso de Nuestro Jomer is a wonderful piece of storytelling, and I love both the animation and Johnny Cash providing the coyote's voice. The Springfield Files was really smart when it aired, capitalising on the popularity of The Simpsons and The X-Files to create fun for all the family, the songs in Simpsoncalifragilisticexpiala(Annoyed Grunt)cious are incredibly catchy and memorable, and it is a great homage to Mary Poppins.
I could go on, but you get the picture!
