The funniest episode of The Fresh Prince of Bel-Air is probably the one guest starring William Shatner. Unfortunately, there is not a good video online of the scene where Shatner, Will, and Carlton are at the dentist high on laughing gas. However, there is a good copy of the bar scene, which causes that visit. While the scene is not hilarious as the dentist one, it is still funny.

http://www.dailymotion.com/video/xlfm6y_william-shatner-1996-fresh-prince-of-bel-air-06×22-eye-tooth_shortfilms?search_algo=1