I wouldn't really say I have a "favorite," since most of the videos people post on YouTube are absolutely crap.
But, occasionally you find some gems. Here are two recent videos I enjoyed.
"Harlem Shake (Horrific Car Accident Edition)"
I dislike the harlem shake meme, but this one actually made me "lol"
Edit: removed video link due to profanity
(Mod Note: The newest video by Biting Elbows is indeed amazing, but a little too spicy to linked directly in the forum. )