It's hard to say who my favorite guest stars were because most if not all I first seem them on The Muppet Show. However my favorite episodes would probably be the later themed episodes when the show had a theme. But mostly my favorites is when they get not such a normal typical happy guest on the show who either messes up the show more than The Muppets or hates being on the show.
Spike Milligan (Is probably my #1 favorite because it's so politically incorrect through out and Spike is just plan silly and surreal and for the first time, all The Muppets play straightmen. lol Plus British Humor is so hilarious regardless. lol, I loved how he kept ticking off Sam the Eagle and Kermit at the End. lol)
John Cleese (Jack Black would later get the same treatment in the new movie, but here's one of the members of the craziest TV show ever Monty Python's Flying Circus. "There is no way he'll do a song, we will stop this at once, he hope this is funny" lol Love the ending lol)
Mark Hamel (Him singing with Angus McGarnagle still cracks me up to this day lol)
Carol Burrnett (She's just plan irritated of The Muppets "Just knock it off you bunch of turkeys!" lol and the ending with "I was met for dancing" some of the best Muppet Show music EVER! And of course the challenge of wearing Animal out. lol Hilarious. lol)
Zero Mostel (Last thing he ever did, When he was doing all that funny stuff behind Sam the Eagle's back, probably one of my top 5 favorite moments on The Muppet Show EVER lol, I bet I can't watch that clip with a straight face lol)
and of course
Marty Feldman (One of my all time favorite episodes. And especially the end with the Sesame Street cast. I remember I showed that episode to my best friend a year or two ago and he was totally confused "Sesame Street? What are they doing there?" and I told them, they're Muppets too but from a different show. lol, Just seeing and Early Count singing to "Hot Time in the Old Town Tonight" still cracks me up. late 70s Count was so funny. Before I go off subject, I just want to say that The Count goes shopping, first day at school and his Rocket sketches still crack me up lol)