I think it'll always be Dallas' realisation of the gravity of his brave but badly thought out insistence of going into the ducts to flush the creature out.
I remember my belly getting that falling feeling as his expression and voice slowly changed to barely controlled panicky terror as he realises he's completely alone and in the cramped confines of the ship's bowels with something he can't even comprehend and his mind is playing tricks on him.
I always felt that the alien showing up was a bit of a let down, at least compared to the insanely tense build up as all his bravado starts to evaporate