It’s gotta be Predator.
No film has ever even approached the amount of testosterone-per-frame level of Predator. You can hit puberty early just by watching it.
Almost every line in the movie is quotable.
There are slow parts, but they’re suspense-slow, not bullshit-drama slow.
And it’s not really campy aside from the testosterone levels - it holds up pretty well as a serious action/suspense movie.
Lots of good ideas here. Still too much plot in a lot of them. 
Fifth Element, True Lies, Terminator 2, Raiders of the Lost Ark are longtime favorites of mine.
Added to my movie to get list are Shoot Em Up, The Rundown, Bourne Supremacy, The Last Boy Scout
If you liked Lethal Weapon and end up liking The Last Boy Scout, also get The Long Kiss Goodnight. It’s another awesome action movie with plenty of humor, from the same screenwriter, Shane Black. It stars Geena Davis (who is smokin’ hot), Samuel L. Jackson (playing one of his earlier badass characters), and the always-reliable Brian Cox.
Shane Black also wrote the excellent Kiss Kiss, Bang Bang, but that’s more of a tribute/deconstruction of film noir and buddy movies than action movies. Might not be what you’re looking for here, but I highly recommend it as well.
control-z, I actually found *The Bourne Supremacy * boring and slow-paced, but that was the middle chapter. The first and third movies, The Bourne Identity and The Bourne Ultimatum, are both a lot better-paced, with a lot more action.
I cannot recommend John Woo’s The Killer and Hard Boiled highly enough, and Woo’s first American movie, *Hard Target * (yes, with Van Damme), is also well worth seeing if you’re an action movie buff. It’s set in New Orleans (and the outlying Louisiana bayous), with Lance Hendricksen and Arnold Vosloo playing evil hunters who hunt homeless Vietnam veterans for fun and sport. Van Damme has a mullet and kicks all kinds of ass, and so does Wilford Brimley as his drunken Cajun uncle.
For all its impossible plot devices, Woo’s *Face/Off * (with John Travolta, Nicholas Cage, and the always-sexy Gina Gershon) is a nonstop thrill ride of an action movie as well. It doesn’t seem to be a favorite around here, but I enjoyed the hell out of it.
The Bourne movies all suck. They’re filmed with shaky-cam and cause motion sickness.
IMNSHO, the best action movie to date has been Terminator 2.
Hot Fuzz.
I rest my case.
It’s a comedy, a tribute to (and parody of) action movies. Not what the OP was looking for.
(Plus, I really didn’t like it, didn’t laugh, and don’t see what all the hype was about. Pit away…)
Léon/ The Professional ought to get a mention.
But it’s not an action movie at all. It has 2 or 3 relatively short action scenes and is mostly a character piece.
No guns, no cars blowing up, no sex but for pure butt kicking action, Jackie Chan’s pre - Hollywood movies (Snake in the Eagle’s Shadow, Drunken Master I and II etc) take the cake. The fluid (and funny) fight choreography in his movies is the very best ever (possibly with the exception of Jaa’s fight scenes).
My vote is for Die Hard. Honorable mentions go to Terminator 2, Raiders of the Lost Ark, and the Bourne movies.
I liked the soundtrack in Aliens. I loved it in Wrath of Khan 4 years earlier. I made the mistake of watching the two back to back years ago to compare the soundtracks, and wanted to duct tape shut all my orifices shut.
+1.
Especially the awesome scene where they freak out and start firing weapons into the jungle in a spray pattern for about two minutes straight. Rockets, machine guns, the works. Branches, trees, underbrush, probably random innocent jungle animals are pulverized. It only stops when Arnold’s chain gun runs out of ammo and continues spinning and clicking for another few seconds.
Then… “Did we hit it?”
Starship Troopers is another one, for me.
They’re both James Horner. It’s like saying the soundtrack to every Jerry Bruckheimer movie sounds alike because they all have Hans Zimmers distinctive style.
GET TO DA CHOPPA!!!
I can’t believe no one has mentioned Under Siege yet. It’s got everything an action movie needs:
Steven Segal
Topless Baywatch babe Erika Elaniak
Evil Gary Busy
Crazy Tommy Lee Jones
Not a perfect movie, but I also liked** Exit Wounds**.
But **The Rock ** has to be one of my all-time favorites. No nudity but you have the following:
Improbable premise - terrorists take over Alcatraz and only some former SAS agent who was the only person to escape from the island and a chemical expert who has never seen combat can save the day
Gratuitous car chase - Ferarri vs Hummer
Musical score by Hans Zimmer
Comic buddy action
One liners
Great villian in the nobel but misguided Ed Harris character
Usual cast of assorted badasia and goonery - Michael Biehn, John C. McGinley, Tony Todd and others who you probably recognize, even if you don’t know their names.
There’s also a noticeable lack of airplane hijack movies here:
Executive Decision
Passenger 57
Air Force One
Snakes on a Plane
No, in the case of Aliens, Horner (under extreme time limits as described above) pulled entire arrangements out of his previous score for Star Trek II: The Wrath of Khan. They don’t just sound somewhat similar (like John Barry’s compositions); they’re virtually identical. This is particularly noted in the sequence where Ripley is escaping from the Queen’s nest, which borrows the arrangement from the “Enterprise v. Reliant” scene at the end of the latter.
Stranger
Raiders of the Lost Ark
Predator
Lethal Weapon
and Point Break were all calls I embraced, but I’d have to go with Raiders for #1. Non stop, creative, original and just a walloping good tale.
I remember when it and Star Wars came out between the late 70s and early 80s and thinking ‘Bloody hell but we’re in for an amazing new and altogether thrilling age of filmmaking.’
None of these even attempt to be “believable”, but I thought you wanted action 
Planet Terror
Judge Dredd (which I actually like, except for the side-kick)
Sin City
Battle Royale
And I would like to include Yojimbo, even though it has its slow moments, just because it has the most bad-ass tune when it introduces the “hero”.
The Matrix is my vote.
Guys. Seriously. Have you seen Hard Boiled, mentioned earlier? Jesus, that movie is insane. The final gunfight is about 40 minutes long, and the shooting never stops. It’s like a chainsaw to the balls.
IMDb lists the body count as 307. Three hundred and seven! It’s a two-hour movie! That means, on average, more than two people get killed every minute! Also, they fired 100,000 rounds of blanks during the filming. And this is a cop movie, not a war movie! Top that, suckas!
I also thought of Hard Boiled. I read a review of it once that described the last scene as “the final scene of Taxi Driver stretched out to almost an hour,” which was pretty accurate. I’d still have to give “most perfect” to Raiders, though…maybe give Hard Bolied “most intense” or “most actiony.”
I work in the video game industry, and thus have plenty of people who are into all things Asian and nerdy and so forth. So I rented Hard Boiled, and my basic impression was… meh.
Sadly, I don’t even remember enough to say WHY it was meh. But I certainly vastly prefer something like Die Hard, or several other genres of Asian action movies (the Jackie Chan genre and the Crouching Tiger genre, in technical terms).
Maybe I’m missing something, but why not The Wild Bunch, Zulu, or even The Great Escape? And, isn’t Saving Private Ryan an action movie?