I've always onsidered "Alien" so much SciFi that it never flew into the actin radar, but I guess it could. Especially "Aliens" with all the Space Marine stuff and Ripley manning the loader to take on the queen. "Get away from her, you BITCH!" Excellent line. Excellent movie. Lethal Weapon, "eh". Predator, one was good, two was sort of "eh".
Nobody has mentioned any war flicks at all. Tora! Tora! Tora! has, to date, the best attack on Pearl Harbor sequence, although there are elements to the battle that have come out in the nearly 40 years since it was made so aren't included (like the Japanese minisubs which, while not having a real effect in the battle, did provide some warning as to the impending attack).
Pearl Harbor, while a stinker with too much romance crap, did an excellent job of recreating the attack as well, including some historical elements not readily known during the filming of TTT, and some great true personal elements (the actions of Petty Officer Doris Miller played by Cuba Gooding, Jr. for instance). Although the movie as a whole was definitely another Michael Bay stinker...
Let's see: A Bridge Too Far, The Great Escape (a personal favorite of mine), The Dirty Dozen, Kelly's Heroes, Platoon, Apocalypse Now, Saving Private Ryan (if just for the Omaha Beach Landing), Midway, Tora Tora Tora, and The Guns of Navarone... I think that covers the best of the action/war movies.