Any of the moments from this episode. They really picked the perfect voice actor for such a character.
In fact, nearly half of the moments in this ENTIRE SERIES are favorites of mine. Courage the Cowardly Dog had such a powerful way of delivering pure nightmare fuel to children, and they managed to get away with it for 4 long seasons too. I think it's more than partly responsible for horror being my favorite genre.
It also had a good occasional dose of mentally scarring psychological terror (See Below)
RUNNER UP:
I never saw this on TV, but I really like it. Not only because I love that song, but because the visual style is so surreal, especially for an old cartoon.
And the fact that the long-legged thing is actually voiced by Cab Calloway helps.