I was just thinking about it, so I figured I'd start a thread. Here are my picks:
Bruce Forsyth: I've been planning to write an article about him and his episode for Tough Pigs, but my main points are: he's goofy, grating, insincere, a show-off and a phony
Marisa Berenson: not an insult to Ms. Berenson per se, but her appearance on the show left a lot to be desired. What exactly was her first number supposed to be showcasing? And in regard to her song, is that really how she sings? No closing number, either. Still, she was good in "Cabaret" and IMO one of the most beautiful women in the world. Nothing to sneeze at, that.
Sylvester Stallone: I don't really need to explain this, do I? Suffice it to say, his appearance on the show was more of a novelty than a display of his versatility as a performer.
Shields & Yarnell: first number- excellent; second number- unimpressive; closing number- is it over yet?
Arlo Guthrie: nothing against his musical numbers; they were all fine, and his closing number was lovely, I thought it was a great rendition of a beautiful song. But overall, he seemed kind of awkward, like he didn't belong or didn't really know why he was there. There's a nice moment at the end with Miss Piggy, and it's one of the only times in the show where it seems like he's actually there; the rest of the show he seems like he's distracted by something.
Anne Murray: her singing was great; her acting was not
Jonathan Winters: not very funny, seemed to not enjoy being there; no closing number.
Mark Hamill: Oy. The 'cousins' bit got old fast. Hamill was probably the only guest star who was least believable as himself.
Joan Baez: her songs were good, but the problem with her guest appearance was the same as it was with the episode itself: both were TOO PREACHY
Brooke Shields: her limitations as a performer were on full display. She's improved somewhat over the years, and as she demonstrated at the live Muppet Show, she can sing. Still, when I first saw her numbers on TMS as a lad I fell in love with her, so my cynicism can only go so far.
Senor Wences: I dunno, I was bored with his bits on the show. He was a more skilled ventriloquist than Edgar Bergen but not as funny.
Wally Boag: more corny and annoying than funny
Debbie Harry: same as Anne Murray; singing was good, acting was most certainly not; 2nd to Mark Hamill for not being believable as herself
Side mentions:
Valerie Harper: clearly enthusiastic, loved being there, loved the Muppets; couldn't sing or dance, a big minus; good impersonations, though
Zero Mostel: underused; sometimes he seemed to be doing his own thing rather than playing off of his co-stars; acting 'at' instead of 'with'; they should've given him more to do and given him a scene with one of the core Muppets: Kermit, Piggy, Fozzie, or Gonzo
Judy Collins: non-interactive; like Mostel, she should've had a scene with Kermit or Fozzie or other regular characters, maybe a dressing room scene would've done the trick
What are yours?
David "Gorgon Heap" Ebersole