Die Hard (1988)
Beyond that, Die Hard is arguably the quintessential action movie. At the very least, it's the quintessential action movie of its generation, and it could easily be the best action ever made.
It's been a few years since I last watched it, and it holds up marvelously. Bruce Willis and Alan Rickman are both amazing in their respective roles as the ultimate cop and robber, and they've got great material to work with, thanks to some fantastic writing and directing. Decades of knock-offs would follow this, but none would figure out what made Die Hard work as well as it did.
There's a lot that contributes to the film's success. While it's by no means the only reason the movie worked, the element other films seem to have the most trouble duplicating is the villain. Hans Gruber is simultaneously likable and repulsive. He's brilliant: technically, I don't think he made a single mistake in the entire movie. He's methodical, logical, and driven. To overcome him, John McClane has to be better.
In addition, while Die Hard isn't remotely realistic, it shows a hell of lot more restraint than the vast majority of its imitators. We never see John McClane take on dozens of bad guys. In fact, on a scene-by-scene basis, he hardly ever fights more than one or two at a time, and when he is confronted by more than a single opponent, he flees. Even one-on-one fights have real suspense, thanks to the director remembering something few others would after him: that these guys are supposed to be professionals.
If you haven't seen this in a while, I strongly recommend doing so. And, needless to say, if you somehow never got around to watching Die Hard, this one is highly recommended.