What is your absolute favorite guest star in a sitcom?

Mine are:

~ Robin Williams on "Happy Days" ("My Favorite Orkan")

~ Lucille Ball on "Here's Lucy" ("Lucy Carter Meets Lucille Ball")

~ John Wayne on "I Love Lucy" ("Lucy and John Wayne")

~ Harpo Marx on "I Love Lucy" ("Harpo Marx")

~ Fabian on "Laverne & Shirley" ("Laverne & Shirley Meet Fabian")

~ Milton Berle on "Here's Lucy" ("Lucy and the Used Car Dealer")

~ John Ritter on "The Mary Tyler Moore Show"

~ Mary Tyler Moore on "The Ellen Show"

~ Cloris Leachman on "Malcolm in the Middle"

~ Redd Foxx on "Sanford & Son" ("Fred Meets Redd")

~ Carol Burnett on "The Lucy Show" ("Lucy Gets a Roommate"/"Lucy & Carol in Palm Springs")

~Danny Thomas on "The Dick Van Dyke Show" ("It May Look Like a Walnut")

~Danny Thomas on "The Lucy-Desi Comedy Hour" ("Lucy Makes Room For Danny")

and probably many more . . .