Aliens is my favourite. Going back to this "realism" thing, I don't enjoy watching any film where a group of untrained civilians with no weaponry has to survive/compete against a formidable foe, especially one of the ultimate bad asses of all time (aliens). I love Aliens so much (even out of most sci-fi/horror films in general) because you have got a group of highly trained marines with all the weaponry they want trying to kick ass and survive. They do their very best and still don't make it but it is still more realistic a story for me to watch. If you have got a group of civilians or scientists up against them then I don't need to watch the film because I can tell you what is going to happen
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Although the more recent AvP and Predators films got a hammering I love them purely for this reason, especially Predators. You have got a group of people put together specifically to give the Predators a run for their money.