For me, it's probably this:

- Shawshank Redemption - Whenever I see it's on I watch it. Period. It doesn't matter what time it is, how far along the movie is. I'm in. The last 30 minutes are the best 30 minutes ever.
- Bourne Identity - I know this is silly, but it changed the action genre. Prior to Bourne the action movie looked like the cheesy 90s Bond films with too much lighting, too much staging etc. After Bourne the handheld, raw, insane fight scenes of people using objects near them became the norm. The new Bond films, the rebirth of Mission Impossible and even movies like Taken and other action films just don't happen without Bourne. Plus it's amazingly rewatchable.
- No Country for Old Men - I just love this movie. So tense from start to finish.