Alien for me, for a multitude of reasons.
It was one of my earliest exposures to what film really can be, just a mind blowing, heart felt story and at the heart of it are these non-stop, inhuman killing machines.
Despite the fact that they are the more obscure looking and less humanoid of the two, for me, in some way, they just seem to be more believable. Maybe it's the fact that they move in such random and unusual ways and for me, an Alien wouldn't be humanoid in its look and movment. It's Alien afterall.
Preds, seem to have a heart at times. No, wait, here me out. Predators...now in my mind, a Pred should never want to help someone escape a planet, wethere they've been freed by them or not. They should be more Hey, Thanks, before blowing his guts for miles around. Similarly, the trap using Danny Trejo as bait. Pred shouldn't even begin to comprehend the idea of helping your fellow man, so they shouldn't have even been able to conceive a plan like this. Just mow the bastards down where they stand.
The amazing interaction between Aliens, The Queen and Ripley....that's just absolute gold in my mind. That is something that is never gonna be able to be replicated, copied or even bother to be attempted in a scenario like this again. Superb, superb cinema.