Gladiator gets my vote, it's a masterclass in acting, music, cinematography, editing, and direction. Crowe and Phoenix are superb, the action is razor-sharp, and it might just be Hans Zimmer's finest score which is saying a lot.

I also really love Alien and Thelma & Louise, and Black Hawk Down and The Martian are excellent as well.

My ignorant vote goes to gladiator, I say ignorant because I really haven't seen many of his films. But Gladiator was  such a great film that it is one of my all time favorites. One of the things I remember most about the film is it's music. It is amazing! It introduced me to Enya. Also I remember many quotes from  the movie, which means that I was paying attention.

I'll throw in the Directors cut of Kingdom of heaven. If it wasn't for the historical inaccuracies, it would be a 10/10 in my book.

Speaking of Ridley Scott; I'm currently watching Raised by Wolves on HBO. Only 3 episodes in, but I really like it so far.

I didn't (and won't) vote, because I saw Gladiator too long ago and I want to rewatch it. Also, because there are at least two other movies on that list (Thelma & Louise, Hannibal, maybe The Martian) that I wanna watch at some point. But, as for now, I'd say Blade Runner > Alien > American Gangster - which I also think deserves a mention.

[Edit]

Stupidly funny fact: I've just seen that I actually voted.

I highly recommend Thelma & Louise and The Martian, both awesome films. 

I've never actually seen American Gangster, maybe I should check it out.

Copy that, I'll probably watch The Martian too.

American Gangster doesn't add much to the genre, but what it does it does well.

Can't really decide between Alien and Blade Runner but I give it to Alien just because not many achieved it to make a horror movie (with monsters/aliens) which is not only an all time classic but also considered as one of the best movies of all time. To achieve that is pretty good.

That Cameron achieved almost the same with "Aliens" is pretty decent as well

I'm going with Blade Runner, and I didn't even like this film much until recently, now I love it. I think I might have been seeing a bad version of it before, because I just didn't like Blade Runner. I watched it again after 2049 and loved it.

Alien was good, too. But Aliens, in my opinion, is a far more enjoyable film. But I'll have to watch the two back to back again, it's been a long time.

Honestly, while I loved 2049, I never liked the original Blade Runner much.

There's a lot to admire about it, like its set design, atmosphere, themes, and Hauer and Hannah's perfomances as the replicants.

The problem I felt was in the two lead characters. Decker and Rachael were just really flat to me and neither likeable nor interesting.