Okay, a longer list it is:
Juliet Prowse, Connie Stevens, Joel Grey (love the look and feel of those first few)
Rita Moreno (so many different comedy styles in the same half hour! So terrific!)
Paul Williams (my all-time favorite, the template for things to come, the format clicks in)
Ben Vereen (a classic, great guest star, memorable plot, great pacing & look!)
Vincent Price (monsters! also a great look)
Don Knotts (first episode I had on tape; funneeee!)
Rich Little (the humor, look and feel of it- oh, and monsters!)
Teresa Brewer (great pacing, colorful and energetic episode)
George Burns
Bernadette Peters (the second season's 'blue' period; monochromatic gray/blue look is neat, show is sentimental at times and funny at others; this one is a classic!)
Rudolf Nureyev (one of my all-time faves! Art and culture, indeed lol! Dignity and class just makes things funnier)
Elton John (this was where my Elton John fanaticism started)
Lou Rawls (my favorite singer! Also a nice colorful, fast-paced, funny episode)
Cleo Laine (love Cleo's numbers, and Fozzie's weaseling his way into every part of the show)
Julie Andrews (classic! Julie works so well with the Muppets)
Peter Sellers (the plot is a little forced, but Peter is brilliant!)
Petula Clark (great second season show, great songs from Petula and classic acts all around)
Roy Clark (plot is hilarious! Roy is terrific!)
James Coco (fun episode)
Helen Reddy (fun episode, great music, esp. "You and Me Against the World" and "We'll Sing in the Sunshine"; great visual design, too)
Harry Belafonte (this one has a different look and feel to it, but it's kinda cool)
Lesley Ann Warren (this one's just enjoyable, nice to look at- so is Lesley, btw... Grrrrowwwwl!
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Spike Milligan (the only guest star crazier than the Muppets; funny and wacky and frantic, I love the look of the show and also how the theater is so full of characters)
Roger Miller (a classic!)
Lola Falana (touching while still being funny and sardonic at the same time- brilliant balance of humor and pathos, some of the best writing they've ever done on the show. Love "He's the Greatest Dancer" and "United we Stand", too!)
Lynda Carter (fun, great music, neat look to the show)
Alan Arkin (switches between different humor styles, great, fun episode)
Loretta Swit (a classic, fun, funny and well-written; great musical numbers, too)
Glenda Jackson (I just love pirates, also I love the look and feel of this show and the acts in it)
Roger Moore (saw it years ago, love the look and feel of this one too, love "In the Navy" and the various sketches)
Paul Simon (all hail Paul Simon!)
Gladys Knight (she's such a great singer, show had some great numbers)
Hal Linden (different but fun and funny; Hal's opening number is priceless, also he's just very talented)
Marty Feldman (one of their most perfect guest stars, hilarious! Arabian Nights was funny, innovative, and looked great too)
Chris Langham (one of their funniest guests)
Gene Kelly (a fitting close to one of the greatest TV shows ever)
David "Gorgon Heap" Ebersole