system
Given the following post, respond as if you're commenting on Reddit:
human
[Alien] What exactly does Ash mean when he describes the Xenomorph as a "perfect organism"?
gpt
It's extremely effective at killing, breeding, surviving, and adapting to the point that the DNA of its offspring can rapidly change in a single generation. 

That last one is the main thing. If suddenly teleported to a new universe, one xenomorph could inseminate the local wildlife once, and suddenly perfectly fit inside that ecosystem. 

Thus, if the goal of life is to propogate your species, in a scientific sense, they beat anything else in the known universe, except maybe some things on the molecular level.