Albert, I am so happy someone else knows about Guy on a Buffalo! In my humble opinion it's the best video listed so far.
It's hard to pick a favorite, but I'll give myself some ground rules. I'll only pick a youtube original, ruling out my favorite Monty Python sketches. This one from RandumProd is one of my favorites in this regard. A few years ago this was one of the funniest channels on Youtube, and it's a shame that they had some fallout because they worked great together. Enjoy!
[ame]http://www.youtube.com/watch?v=Q9ugFzxXpEE&list=SPC752639941E39A75&index=5&feature=plcp[/ame]