Disposing of the Newborn via a hole in the spacecraft window and vacuum pressure effects.
Purvis killing Wren with his own chestburster.
More of a behind-the-scenes version of this, but when Ripley 8 threw the basketball over her shoulder and scored a perfect hoop? No tricks, Sigourney Weaver actually pulled that off!
Sigourney Weaver says she regards making that basket as her own personal Crowning Moment of Awesome... well, after her wedding and the births of her children, anyway.
"Father's dead, asshole!"
The alien in the ice spray room. It learns, after one go, that the ice sprayer is triggered by the button. It not only learns to stop acting up when the hand is hovering over the button, but it also realizes When positions are reversed, that it can do the same. This is the first time we are shown just how smart these creatures are. While it has always been implied through their actions, this time we see that, in spite of the fact that they're incapable of communicating with humans, have no known written or verbal language, and are Always Chaotic Evil, they may very easily be as smart as humans.