Ok, so I'm one of the people who like to see the alien as a perfect organism, Alien is my favourite film of the series and I love the way it's portrayed there. But I would never use the Ash quote as an argument to prove that the alien is the ultimate creature or to prove a point in one of the never-ending Alien vs. Predator discussions.
Because "Alien being a perfect organism" has little connection to what Ash says, really. He just describes what we see. That's what is far more important: the way it is portrayed in the movie and the role it plays. In the history of the Nostromo, the alien indeed appears to be an indestructible, ultimate predator, pure instinct, a killing machine. In this case it's the form, not the substance that matters.
You can say all you want about the Nostromo crew being unarmed, and Ripley ultimately killing the alien by herself, etc. etc., and from a logical and rational point of ciew you'll be right. BUT, from the narrative standpoint, the alien is something far more than a dangerous animal. He's something almost (?) supernatural, a being not subjected to laws of physics and nature. And I think that's why the movie worked so well - a completely realistic and believable spaceship representing the world of logic and science and it's crew suddenly encountered something that defys all laws of science and reason, a creature of almost demonic characteristics.
Later this approach was dropped and in my opinion, cheapened the whole concept and made the alien definetely more animal and by that more "realistic", "logical", and safe. And pairing it with the Predator ultimately sealed this rational and animalistic image. The designs reflected this idea, too, turning from biomechanical to fleshy.
So, in the end, it all depends on your tastes: a "lovecraftian" beast incomprehensible to human thought or just an extremely dangerous animal from outer space. I prefer the former.
