9. David Duchovny And Gillian Anderson As Fox Mulder And Dana Scully In "The Springfield Files"
When a drunken Homer spots an alien in the woods near Springfield, who shows up to investigate? Why, the two FBI agents assigned to handle cases deemed too unusual for the bureau at large -- open-minded believer Fox Mulder and scientific skeptic Dana Scully. David Duchovny and Gillian Anderson reprise their roles from the hit Fox drama, albeit this time in cartoon form. This isn't the only time live-action characters made the jump to The Simpsons (Jack Bauer and most of the ensemble of Cheers pulled off similar stunts), but it was the best. Mulder and Scully eventually leave Springfield in exasperation, but Lisa discovers the true identity of the supposed alien -- Mr. Burns. Also, look for a silent cameo from The X-Files' main baddy, the Cigarette-Smoking Man!
Best line: "But somewhere out there, something is watching us. There are alien forces acting in ways we can't perceive. Are we alone in the universe? Impossible. When you consider the wonders that exist all around us... voodoo priests of Haiti, the Tibetan numerologists of Appalachia, the unsolved mysteries of... Unsolved Mysteries. The truth is out there."