I had no idea who Leo Sayer was until he started singing "You Make Me Feel Like Dancing," during which my twenty-year-old, too-cool-for-health little brother scoffed at me, and proclaimed it "the best song ever!"
Alice Cooper managed to exude serious creepy-vibes. He interacted very well with the puppets, and I didn't have any trouble with the Faust theme, but I kept getting the feeling that he resented how the association eroded his menacing aura. The Muppets didn't make fun of him (they mentioned "Alice" once, maybe) but I still came out of that episode fearing for Kermit's extended future.
Other pieces I thoroughly enjoyed: performing in the train station, Raquel Welch, Jean Stapleton, the Camelot medley of musicals, and Pearl Bailey's unstoppable voice.