I have to break it down by genre...and I'll limit myself to three each - which mean some REALLY great movies are going to be left out.

Drama's:
Gandhi
Patton
Goodfellas

Comedy:
Planes, Trains and Automobiles
Zoolander
A Fish Called Wanda

Fantasy/Sci-Fi:
The Terminator
Lord of the Rings: Fellowship of the Ring (extended edition)
The Empire Strikes Back

Action:
Die Hard
Dirty Harry
Kiss Kiss, Bang Bang

Horror:
Rosemary's Baby
The Shining
Misery

Westerns:
The Good, The Bad, and the Ugly
Unforgiven
The Outlaw Josey Wales