I know it's NOT a sitcom (am I being a corrupting influence ), but Tim Conway and Harvey Corman (sp?) on the old Carol Burnett show. I recently saw this clip of them in a dentist sketch that I found hilarious. http://youtube.com/watch?v=Q9T8i4FkNVo
And who could forget Carol's defining moment as Scarlett wearing the green velvet drapes?? Can't seem to find a clip of it, but it was unbelievably funny.