Oh god, where to begin xD.
Alien, Aliens and yes, even Alien 3 deserves a mention here. Alien is just an outstanding sci-fi horror film with a great cast and monster, props to H.R. Giger for creating such a nightmarsih and symbolic monster. Speaking of symbolic, the themes the film provides is worth looking into as well. Aliens proves that a sequel can kick equal ass, perhaps greater, by taking the franchise into a different direction. This time, it's war. But this isn't a film about space marines kicking ass, no, it's about space marines getting their asses kicked. Over-confident soldiers being beaten ring any bells? Alien 3 isn't as good as the other films, but I still like it because it returns to the grim roots of Alien. All hope is lost here, and it's fantastic to watch. Plus Lord Tywin from
Game of Thrones makes an appearance

.
The Thing is another sci-fi horror film I love. The special effects are just amazing, beautiful even. Every incarnation of the Thing is terrifying and unique, but it's not just the thing that makes this film so great, it's the sense of paranoia of who the monster could be. You never know who is the monster until it's too late. The cast is great too, they never fail to deliver the moments of tension. So is the setting, Antarctica is a great place to have such a film because the environment is trying to kill them as well.
Starship Troopers is another sci-fi military film I adore, for the same reason as Aliens but on a much bigger and comedic scale. It's just one big piece of propaganda to recruit more soldiers. Would you like to know more?
I kinda rambled on there, but I wanted to explain my choices in a bit more detail this time.