Updated:
Season 1:
Indira Stefanianna Christopherson (circa Juliet Prowse)
Elizabeth Montgomery (circa Connie Stevens)
Charro (circa Ruth Buzzi, Rita Moreno)
Bob Denver (circa Jim Nabors)
Bobby Sherman (circa Florence Henderson, Paul Williams)
Gene Wilder (circa Paul Williams)
Tennessee Ernie Ford (circa Charles Aznavour)
Carroll O'Connor (circa Harvey Korman)
Rip Taylor (circa Lena Horne, Peter Ustinov)
Burl Ives (circa Bruce Forsythe, Sandy Duncan)
Carly Simon (circa Sandy Duncan, Candice Bergen)
Shirley Jones (circa Candice Bergen, Avery Schreiber)
Nipsey Russell (circa Avery Schreiber, Ben Vereen)
James Caan (circa Ben Vereen)
Barbara Eden (circa Vincent Price)
Rodney Dangerfield (circa Twiggy)
Michelle Phillips of The Mamas & The Papas (circa Twiggy, Kaye Ballard)
Billy Joel (circa Mummenschanz)
Season 2:
Groucho Marx (circa Don Knotts, Zero Mostel)
Mel Brooks (circa Zero Mostel, Milton Berle)
Sid Caesar (circa Milton Berle, Judy Collins)
Tim Conway (circa Rich Little, Nancy Walker)
Lee Remick (circa Edgar Bergen)
Shirley Temple (circa Steve Martin)
Buddy Hackett (circa Madeline Kahn)
Jack Gilford (circa George Burns)
Vicki Lawrence (circa George Burns, Dom DeLuise)
Kenneth Mars (circa Dom DeLuise)
Red Skelton (circa Bernadette Peters, Rudolf Nureyev)
The Electric Light Orchestra (circa Elton John)
Shari Lewis (circa Elton John)
Tom Jones (circa Lou Rawls)
Christopher Lee (circa Julie Andrews)
Carole King (circa Peter Sellers)
Henry Gibson (circa Petula Clark)
Karen Valentine (& Biff & Sully) (circa Bob Hope)
Paul Lynde (circa Teresa Brewer)
ABBA (circa Cloris Leachman)
Season 3:
Harry Chapin (circa Leo Sayer, Roy Clark)
John Lennon (circa Gilda Radner, Pearl Bailey)
Seals & Crofts (circa Jean Stapleton, Alice Cooper)
Johnny Rivers (circa Marisa Berenson, Raquel Welch)
Flip Wilson (circa Raquel Welch, James Coco)
Jose Feliciano (circa Helen Reddy)
Barry White (circa Harry Belafonte, Danny Kaye)
Billy Davis & Marylin McCoo (circa Danny Kaye, Spike Milligan)
Leon Redbone (circa Elke Sommer)
John Ritter (circa Lynn Redgrave)
Season 4:
Thelma Houston (circa John Denver)
The Spinners (circa Dyan Cannon, Victor Borge)
Ed Asner (circa Linda Lavin, Dudley Moore)
Neil Diamond (circa Dudley Moore)
Dan Fogelberg (circa Kenny Rogers, Lola Falana)
Olivia Newton-John (circa Dizzy Gillespie, Liza Minelli)
Audrey Landers (circa Jonathan Winters, Christopher Reeve)
The Stars of "Logan's Run" (circa Star Wars)
Barbra Streisand (circa Lynda Carter)
Sammy Davis Jr. (circa Carol Channing, Andy Williams)
Robin Williams (circa Doug Henning, Alan Arkin)
Cher (circa Diana Ross)
Season 5:
Gunther Rebel Williams (circa Gene Kelly)
Elliott Gould (circa Loretta Swit, Joan Baez)
Art Garfunkel (circa Senor Wences, Jean-Pierre Rampal)
Donna Summer (circa Paul Simon, Melissa Manchester)
Kenny Loggins (circa Tony Randall)
Charles Nelson Reilly (circa Mac Davis, Carol Burnett)
David Odgen Stiers (circa Gladys Knight, Marty Feldman)
Richard Pryor (circa Chris Langham)
France Joli (circa Johnny Cash, Buddy Rich)
Rex Harrison (circa Roger Moore)