Saving Private Ryan: One of the greatest war movies of all time. It ventured so far beyond the overused 'Let's shoot Nazis' theme, and the troops were diverse- not just a bunch of Irish Catholics or Italians from New York.
28 Days/Weeks Later: These made me truly fear the unlikely event of a zombie apocalypse. I no longer felt secure in the fact that I could out-powerwalk the living dead, but feel the need to sprint to a car and drive.
The Lost Battalion (remake): The best movie about the world's forgotten war EVER.
Transformers series: Big robots, explosions, and Megan Fox running around. What's a guy not to like?
Star Wars series: I'm actually surprised this wasn't on here.
PS I Love You: Great acting, good (but very sad) plot, and the only chick flick I can actually stand to watch multiple times.
Tropic Thunder: Self-explanatory if you've seen it. Hilarious, over-the-top, and it saved Tom Cruise's career.