Forgot what might be in my top three, The Jonathan Winter's Show. Not the one from the 50s (I was just born and living in Europe), but the program which ran from 1967-69.
Winters' brilliance (often dark and/or demented) can't be overstated. Really amazing sketches. But the weekly segment where Winters would find himself amidst a sea of props, and launch into an insane array of improvised characters, was hysterically funny.
IMO, Winters is possibly the greatest comedy mind America ever produced.