Happy Alien Day, all!
Whew, favourite scene... that's pretty tough to decide, considering all the iconic scenes in Aliens. I could go with the scene I adored even before I saw the whole movie, being the ultimate badass powered armour vs. Queen Bitch fight, which still gives me chills to this day.
But upon subsequent viewings, I'd have to tag the first ambush when the marines enter the hive, the tension that has been building since first landing on LV-426 exploding all at once, horror piled atop horror, leaving the audience breathless almost nonstop for the rest of the film.