(Re-posted from another old thread circa 2013)
Man...I'd have to narrow it down to 5 episodes (I haven't seen all 120, but here goes)
Season 1: Rita Moreno (loved the opening dance number, the panel discussion, and "Fever" with Animal)
Season 2: Elton John (the first full TMS episode I ever saw...plus, Elton is one of my all-time fave singers)
Season 3: Harry Belafonte (Turn the World Around is so uplifting and Banana Boat is one of the most hilarious numbers EVER shown on TMS)
Season 4: Lynda Carter (hilarious superhero plot and Newsman subplot, and Lynda never really managed to keep a straight face for more than thirty seconds)
Season 5: Linda Ronstadt ("When I Grow Too Old to Dream" is the most beautiful song ever done on TMS, in my opinion)
Side note: now that I've seen more episodes since that last post, I also have to add the following episodes:
Season 1: Ethel Merman, Avery Schreiber, Candice Bergen, Florence Henderson
Season 2: Dom Deluise, Madeline Kahn, Julie Andrews, John Cleese, Steve Martin, Peter Sellers, Bob Hope, Petula Clark, Rudolph Nureyev
Season 3: Gilda Radner, Pearl Bailey, Alice Cooper, Raquel Welch, Lesley Ann Warren, Sylvester Stallone
Season 4: Liza Minelli, John Denver, Star Wars cast
Season 5: Paul Simon, Carol Burnett, Gene Kelly
Each one has its own reason for being so special...but I lean pretty heavily on the Season 2 episodes because that's my favorite season.