These are my seven favorites. (Why? Five was too few, and ten was too many.)
1) Elton John. A personal favorite singer of mine, and his episode did not disappoint.
2) Paul Williams. This episode launched Paul's career with the Muppets, which represents his best work (IMHO). His episode is fantastic, too. The classic Beautiful Day Monster sketch is deservedly hyped.
3) Rudolf Nureyev. For starters, Sam features heavily here, so that's a big plus. For another thing, it features my favorite EM sketch ("Minuet in G Major"). Lastly, the finale is my favorite finale. Great episode.
4) Brooke Shields. Seriously though, this was great. Whitmire's rabbit delivered my all-time favorite Muppet Show quote ("Look, buddy. Either ya tell me where the rabbit hole is, or I tear your head off."), and the Alice in Wonderland theme played out well enough.
5) Mark Hamill. The Muppets and Star Wars were apparently a good mix. This featured my second-favorite Pigs in Space ("Who's your tailor? I love that outfit!"), and Angus McGonagall is genius.
6) Gene Kelly. How can you go wrong with an episode heavily featuring Beauregard, an end-of-the-world motif, some great musical numbers, the best Pigs in Space ever ("What, miss dinner? Why, they're having swill stroganoff!"), AND one that features the first and last appearance of my namesake character? You can't.
7) Paul Simon. Let's see here. A wonderful singer who really appeared to kick around the set and enjoy the Muppets more than most guest stars; "50 Ways to Leave your Lover" performed by Floyd, Janice and Rizzo; Gonzo attempting "El Condor Pasa" and subsequently getting his chickens stolen; talking asparagus... excellento.