Even when I made the separation between "most fun" and "favourite" in my brain (not as easy as you might think), I came up with the following:
Kaye Ballard: The Electric Mayhem threatens to quit because they find the theme song square, so Floyd writes a new one, which Kermit hates, leading the EM to split and Rowlf to play the closing theme himself ("Heh-heh. Classic.")
Steve Martin: There was just something special about the Muppeteers supplying their own laugh track to the off-kilter audition episode (complete with vintage SM goofiness - and he plays along to the early bits of the closing theme, which makes him the only TMS guest star to do so if I remember correctly)
John Cleese: Part One - Unwillingness to work with the pigs until his agent gets eaten; Part Two - Gonzo: "Pull yourself together!" "I wish I COULD!" Part Three - Closing-number meltdown: "Right! Where's the frog?!"
Spike Milligan: The "international episode" (pun intended) that was already springing a leak and then went completely off the rails with the never-ending "It's A Small World" finale and the guest star throwing pillows at Kermit
Loretta Lynn: Speaking of "off the rails," the railroad station setting was a TMS masterstroke, especially the off-key opening and closing themes (Rowlf: "No wonder this sounds bad! We're playing a timetable!") and "The Rhyming Song"
Liza Minelli: What's not to love about the Queen of Broadway hamming it up at every turn of this Muppet murder mystery?
Phyllis George: The First (and Last?) Annual Muppet Awards, a delightful Emmy/Oscar spoof that also took a couple of shots at Ms. George's former Miss America status (especially the continuous flower presentations during the closing credits)
Carol Burnett: A guest-star meltdown of Cleese-ic proportions and a wonderful skewering of the general TMS format through Gonzo's dance marathon
Paul Simon: I don't think any other musical guest so cheerfully endured a Muppet mangling of his musical catalogue (and just about every time Gonzo opened his mouth in this episode, I roared)
Marty Feldman: The Arabian Nights theme was already full-bore crazy before the Sesame Street gang showed up for the big finish
...and those are just the ones I can think of, off the top of my head...