Aliens is clearly an action movie. So what if it's sci fi? The last forty minutes of that movie comprise one big action sequence. It has handguns, machine guns,flame throwers, jets, grenades, grenade launchers, noble self-sacrifice, bad guys and good guys getting mowed down left right and centre, hero/villain stare-downs, a countdown to destruction, giant machines, smack downs, call-outs, a hero/villain brawl and a thermonuclear frikking explosion. What more do you bloody want? The movie has ambushes, shit-talking, tanks, marines, plane crashes...I mean, with this movie and Terminator 2 James Cameron basically redesigned and perfected the modern Western action movie. I mean, industry people have been playing catch-up with these films ever since.
Okay, enough with my tirade.
Obviously Aliens and T2 are on my list. I'd also include:
Hot Fuzz (for both roasting and also being better than all the movies it roasts)
The Dark Knight
The Lord of the Rings Trilogy (another example of films people have been trying to catch up to ever since)
Die Hard (obviously)
The Killer
Hard Boiled
The Empire Strikes Back