This troper considers the entire fight scene between the Alien Queen and Ripley to be the origin of his love of mecha.
You thought the MECH was impressive? Just the Alien Queen! This was in the time before CGI everything and that is the biggest fully-functioning monster puppet ever made. And the result? Quite possibly the single most awesome monster ever seen on the screen! CGI just can't replicate that level of terrifying realism!
You know its awesome when Bishop says after the entire fight scene, "Not bad for a human."
"I remember seeing the film, midnight screening on Hollywood Boulevard, and that line brought the house down. People stood up and cheered. It's probably the most gratifying moment of my producing career, was the reaction to that shot." — Gale Anne Hurd
Another moment just before that one: When the alien queen comes out by surprise and splits Bishop in half. After almost 20 years, this troper has yet to find a monster entrance that is more impressive.
And then Bishop follows with one of his own: despite being torn in half, he manages to save both himself AND Newt when the airlock is sucking them out of the ship.
The battle itself is not only awesome, but it's quite rare to see two Mama Bears duke it out in the form of Ripley who was defending Newt and the Queen who is pissed because Ripley and Co. blew up her babies.
As the tank is escaping with the marines, an alien begins to force the door open. Hicks shoves his holdout shotgun down it's throat. "Eat this!"
When she's retreating from the Queen's nest with Newt in her arms, and an egg ominously opens up next to her, Ripley cocks her head to one side, as if to say, "Well, that's it. Now you're toast." and torches the entire room with her flamethrower before blowing the holy shit out of the Queen's egg sack with pump-action grenade after pump-action grenade.
"You always were an asshole, Gorman."
Hudson screaming "Fuck you!!" even as he's being dragged to his death by the aliens.
A meta-example, Sigourney Weaver getting nominated for Best Actress in a sci-fi/action/horror film which both then and now is almost completely unheard of.
Although this troper had seen the film in chunks previously, when it came time to watch the whole thing through, one scene in particular really got him. After being trapped with the facehugger, Ripley signals for help from the marines, and they make their way down to sickbay, where they're momentarily horrified. What does Hicks do to save Ripley and Newt? He jumps through a glass window and starts wrestling with the facehugger to keep it away from Ripley. And what about Newt? She gets saved by HUDSON, who single-handedly shoots her creature to bits. The fact that the latter was proclaiming "Game Over" just a short time ago only elevated the scene's level of badass.
A moment of respect for Newt, a terrified little girl who manages to fight and stave off a creature that's killed grown-ass men armed with flamethrowers.