Ah.. it is a truly open question here. Honestly the aliens were , in aliens perfect but in aliens , they are made weaker and rely on massive numbers if you just watched alien , you'd assume they were not always a bug like race but thought independently. Aliens changed all this , even though it is a great movie it effects the aliens image - from a stealthy organism that just can't quiet die to a horde of monsters ready to devour anything. In alien three and A:R this continues. Then We have the Predators like the aliens , in Predator they seemed unbeatable and only lost by an interesting code of self honor and defiance. But , even though they are shown strong it Predator 2 , they look all to human with a honor system made , even though this was in the first one it kinda wasn't. All we get in the first film was they hunt for sport and do to not a challenge , do not killed unarmed humans. But in Predator 2 they are less of the villain then the drug lords or the greedy FBI guys there. But I'd say predators , aliens are more fascinating but Predators are every persons hero... not but they are my favorites!!
