10. Johnny Cash (S8 E9 - "El Viaje Misterioso De Nuestro Jomer (The Mysterious Voyage of Homer)")
When you are deciding to write a show that involves themes of alcoholism, marriage, soul mates and a departure from the usual Simpsons animation who better to ask to guest star as a talking Space Coyote than Johnny Cash. As Homer trips out on Guatamalan Insanity Peppers he goes on a 'mystical voyage' complete with the mentoring and wisdom of the Space Coyote which helps Homer discover what we knew all along, Marge is his soul mate. It's often cited as one of the best Simpsons episodes of all time due to it's multiple levels of animation, cultural acknowledgements and of course, the Man in Black.