Shawshank Redemption
To me, it is one of the best, most complete movies of all time. At the very least, the best in my lifetime.
A close second would probably be:
The Silence of the Lambs
It used to be my first, but I'm not always in a "psychological horror" mood, so it's not always the "go to" film for me.
I recently got a nice digital projector and I watch movies on a 106" screen in my gaming-cave. Both movies are incredible on the big screen, but especially Silence of the Lambs due to Demme's "up close" camera style for critical dialogue. Shawshank is a period piece, so it's timeless. Silence of the Lambs at times feels dated, and the cinematography (outside of Demme's close-up style) doesn't really stand up anymore.
Also, any classic 70s, 80's or even 90's Sci-Fi. The golden-era for Science Fiction in film probably being late 70's and the 80's. Whenever one of these movies comes on, you're almost obligated to watch it. Stuff like Alien, Blade Runner, Jurassic Park, The Thing.. etc.