After being captured and cocooned, Miller manages to free an arm, grab a gun, and kill the facehugger that lept at him at him.
The Xenomorph known as Grid gets a few. Most notably killing two of the three Predators by herself, the first through a surprise attack from behind and the other through a full-out brawl. And even though she doesn't get into an actual fight with Scar, she's shown to be smart enough to avoid getting killed by him.
Lex manages to kill two Xenomorphs and holds quite a bit of the responsibility for the defeat of the Queen as well. Made especially awesome as she was never armed with anything more useful than a spear. Ripley would be proud.
Scar obviously thought so, too, marking her as kin with Xenomorph blood.
Weyland wielding a home-made flamethrower.
Weyland: Don't you turn your back on me! (fires at Scar).
Lex and Scar working together to fight the Xenomorphs. Never has a team-up looked so awesome.
Lex watches Rousseau loading her pistol.
Lex: Seven seasons on the ice, and I've never seen a gun save someone's life. Rousseau: I don't plan on using it. Lex: Then why bring it? Rousseau: Same principle as a condom. I'd rather have one and not need it, then need it and not have one.
Lex impaling one of the aliens to death as it tries to get her. It was so awesome that it impressed the Predator enough to allow her to accompany him.
When Scar demonstrates exactly what his self-destruct device will do to the nest.
Lex: It's a bomb... I hope it kills every fucking one of 'em!
Later on, we get to see it do just that. Probably the first time one of those had ever been used for a constructive purpose.
In the first movie, right before the first Predator stabs Lex, a tail impales him from behind. Epic music builds up, the Predator is pulled upwards until he's facing a dark tunnel on the wall, and slowly, an Alien (Grid) appears. She leans forward until she's face-to-face with the Predator, and hisssssssses. Also, the first time an Alien and Predator come face-to-face on film.
When Scar finally gets hold of his plasma caster after having killed an alien without it as required by his test. Most of the hive are coming towards him and he just stands there opening fire with the caster, reminding you how deadly a Predator can be. Doubles as a moment for Grid as she dodges the first shot.
Scar saving Lex from the Queen. He jumps, pulls some fancy acrobatic ninja-esque spin in mid-air, and rams his spear through the Queen's head. Say what you will about the first AVP but that was absolutely badass. See here◊
And when the Queen has him mortally injured, cornered and pinned down, she comes down face-to-face to him and hisses in his face. Scar roars in reply
Scar gets one earlier in the movie. A facehugger takes a lunge at him and Scar takes him out in midair with a shuriken. Then an alien quietly slides in from above, and Scar beheads it without even looking. If he hadn't been so full of himself he might have avoided getting grabbed by the second facehugger.
Alien vs Predator: Requiem
Wolf systematically hunting down the Xenomorphs and wiping out any trace of evidence.
There's just something immensely satisfying about seeing the Predalien essentially headbutting it's way out of the sewers, bursting through the concrete and onto the road. Not to be outdone, Wolf then proceeds  his way out of the sewers]].
The fight between Wolf and the Predalien. For one it's actually lit! Second it's just nice seeing the fight the film built up. Hell just seeing the two in their final moments qualify it as a awesome moment.
Dallas taking up the plasma gun Wolf dropped and going to town on the Aliens was awesome to behold.
Special mention has to go to when he takes out the third last alien (the last two were killed by Wolf). Wolf's plasma gun takes time to charge after every one use. After taking out one of the last ones, Dallas is waiting for it to charge up back again. An alien comes up above him, its drool falling onto him. Instead of losing his shit like most people in horror movies do, Dallas averts Too Dumb to Live and doesn't move an inch till the gun charges up. When it does, he hisses "Fuck you" and blows the alien's head off.
The 'origin story' of Six, the Alien you play in the 2010 game. Part of an experiment to acquire more Chestbursters, WY sets up special containers for some chestbursters to be safety contained. However, the WY mooks sent into the lab realize that Specimen Six isn't in its container. The mooks get closer to examine the corpse host, when Six jumps out of the host's mouth. This act impresses Weyland so much that he prevents the head scientist from flash-freezing the lab, saying "I like this one!". It gets even better if the player notices that you see Six burrow back in before the mooks do.
The Rookie from the 2010 game manages to wake up from being passed out, then proceeds to fight his way through the Alien hive, killing the Queen and causing the facility to catch fire. And that's only the second level of his story.
It's hard not to feel like a Big Damn Hero during the final level, when you're charging through the Pyramid to stop Weyland, smartgun ripping apart anything that moves, with the most epic, heroic music in the game playing the whole time.
The player Predator in the 2010 game, "Dark", eventually stumbles upon artifacts of the very first Alien hunt that the Predator's engaged in; retrieving a ceremonial mask belonging to an ancient Predator of great renown, fights his way through a Weyland-Yutani facility in order to retrieve that Predator's wrist bracer, and uses it to destroy the Predator temple that Weyland Yutani's base had been built on; narrowly escaping after killing a predalien.
The 1999 game had a unique mechanic in that each race could shout/roar/hiss, and this affected the morale of the (mook) opponents. With Marines tending to move in groups, a concerted terror campaign by a hidden alien/predator could cause the marines to begin panic-firing. This was especially effective if the marines were carrying flamethrowers or missile launchers. Skilled hit-and-fade alien players or sniping predators could produce the same effects.
In Av P: Evolution, during the Predator Missions, you can encounter Facehuggers which can and Will attach to your face. Game over right? Nothing can pull those things off once their on right? WRONG! If you swipe down when instructed, your Predator will Rip the Facehugger off, slam it into the ground, and Curb-Stomp it! Bad Ass.