Charles Aznavour did a lovely rendition of "Inchworm" (performed by Danny Kaye in a later episode), and had one of the funniest scenes with Miss Piggy that I've seen, on a par with her Roger Moore skit.
Bruce Forsyth was a trip when he helped Fozzie battle his hecklers, Statler and Waldorf. The lesson doesn't stick, though! My favorite number in this one was Forsyth's dance to "All I Need Is The Girl" with a giant bird puppet.
Florence Henderson's episode, at least her numbers, felt a little dated to me. It might have been that she was singing more contemporary songs than most of the guests in that season.
Harvey Korman had an awesome skit with the big blue monster Thog; this episode also has Robin the Frog singing "Halfway Down The Stairs," another of those moments I remember from childhood.
Peter Ustinov did a series of accents and was especially good as the PSSSSychiatrist in the panel skit. Also, Kermit sings "It’s Not Easy Bein’ Green." (I recall he sings it again in a later season.)
The Paul Williams episode is, in my opinion, one of the best they made. I love, love, love him singing "Just an old-Fashioned Love Song" with two muppets of himself.
The Ruth Buzzi episode also features a Robot Kermit.
Lena Horne's episode is another one I remember from its initial airing, because my mother was able to tell me who she was. I am still amazed, watching her sing, to see how much she projects emotionally. I wonder what it's like to meet her in person, if she tunes it down in daily life, or if she's one of those people who's always very charismatic? Like Florence Henderson, she did a contemporary song (Jim Croce's "I've Got a Name"), but I think Lena Horne's flew better. Maybe it's just my taste.