The Amazing: There's a lot to be said for a monster which appears absolutely purpose-built for carrying out its appointed task, and even more to be said if that task is doom-shafting any meat-based life-form idiotic enough to stumble into its (apparently infinite) hunting ground.
Genius Detail: And it goes without saying that we're absolutely in love with the whole face-hugging, stomach-bursting reproduction technique - just delightful, in terms of inventive monster-lore. But a mouth within a mouth? Oh Ridley, you're spoiling us! And look, we're spoiling our underwear.