Wow. This thread really got rolling. In response to some previous comments, I, like most folks, have tired of Tom Cruise, but Top Gun (DTS) is truly amazing for a demo. Not bad as a movie either. Collateral I really enjoyed. The fight scene in the night club was amazing.

I really liked the tangent on movies that are tough to watch, or that elicit powerful emotional responses. To that, I would also add disturbing films that tend to haunt your thoughts for a few days. So here's a stab at some of those:

House of Sand and Fog (the ending)
The Elephant Man
Straw Dogs
21 Grams
Pixote
Deliverance
Eraserhead
Illsa, She Wolf of the SS
A Clockwork Orange
Freaks
El Topo