Well, one of the things which makes the xenomorph so intriguing is that everything we have seen about them makes them biologically feasible. In no way or form do they break the laws of science.
The universe is a very big place, it's no one's say whether a creature like it could or could not exist now or in the future.
What makes a xenomorph frightening is not that it's relentless or unimaginable. What makes them terrifying is that they are comparable to creatures we know today.
An unstoppable creature is not scary because the viewer knows it to be unstoppable.
Also, the only way humans have been able to kill the xenomorphs thus far has been
with science. Humans made the compressed chamber which sucked the xenomorph into the vacuum of space in Alien, the power loader and M41A pulse rifle in Aliens, the molten lead facility in Alien 3, and the entire space station in Resurrection.
The greatest kinds of horrors stem from something which we can associate to things within our own lives. We were terrified from that chestburster breaking out of a human's chest because we are human. We are frightened by the xenomorph because it is humanoid and we are unaccustomed to being prey to a greater predator.
Tack on the elements of stealth and deadly swiftness, and you have yourself the makings of a true horror.