I nearly peed my pants the first time I watched Manos: The Hands of Fate as done by Mystery Science Theater 3000. It probably didn't help that it was late at night and I was by myself...but oh man, Torgo nearly killed me.
30 Rock has had some great moments. When Tracy took drugs before his appearance on Conan and was acting like Chewbacca (among all the other things that happened in that episode) was gold. Also when they showed how all the characters saw each other: Jack seeing everyone with monetary values assigned (and Kenneth only being worth $7), Tracy seeing everyone as himself, and Kenneth seeing everyone as Muppets, and watches Liz walk Muppet-like (as a Muppet) down the hall, and the camera changes and Liz (as a human) is still walking Muppet-like.
ETA: Oh, I have more.

On the Simpsons, the whole sequence which parodies the opening of Raiders of the Lost Ark--with Homer filling the role of the giant boulder--is hysterical.
And the episode of Mr. Bean when he goes to the school, turns some kid blue, is forced to paint a naked lady, and tries his hand at martial arts, and (I think this was the same episode) stole the pants off of another guy who was using the bathroom.