Valentina, given your love of sci-fi, I can't bear the idea of you having only seen the dreadful Prometheus (see here for why I hold that view) and not Alien and Aliens. Please, I'm begging you, watch those two ASAP (ideally in a cinema, but otherwise on the biggest screen and with the best sound system you can find). :)

Predator is good too, but not as good as Alien/Aliens. (I did like Prey - it's a worthy companion piece to Predator).