Here it is, my vote for the single funniest episode of "What's My Line?" I suspect there are some folks out there who will find it irritating how much Groucho demolishes the format of the show around him, but everyone on stage appears to be loving it just as much as the audience is. One of Groucho's finest (half) hours after the breakup of the Marx Brothers as a team.
MYSTERY GUEST: Claudette Colbert (Groucho on Panel)
PANEL: Arlene Francis, Groucho Marx, Dorothy Kilgallen, Bennett Cerf
Groucho returns to WML as mystery guest in 1963: https://www.youtube.com/watch?v=n_NzXFs3Rqg
Groucho returns to the WML panel in 1964: https://www.youtube.com/watch?v=tYSrDfn7z1Q
Groucho returns one last time as mystery guest in 1967:
Groucho Marx on You Bet Your Life: https://www.youtube.com/channel/UCUIbTdEI7D1AMyLATIdKq5w