Lindsey's mini-sub battle with Coffey. He's trying to kill her husband before he can get back to the diving rig, so she proves that Ramming Always Works and slams into the other sub until it shorts out the engine, then knocks it over the edge of the trench, where the increasing pressure literally implodes the front window.
Her bloodthirsty expression just makes it more awesome. You could get the impression she was just waiting for an excuse to get that guy.
The filmmakers emphasize just how powerful Catfish's punch is by including a single white frame in the film as the punch connects with its target. Clever cinematography at is finest.
Some of the more practical stunts and effects, such as the breathing liquid scene; it's easy to imagine Ed Harris' panic as he has to willingly let his helmet fill with the fluid while the crew tries to keep him calm.
On a wider view, the inclusion of the fluid breathing system in the film. It was miraculous technology back in the 80s, and remains extremely impressive even today. That Cameron included a practical demonstration of the technology on screen without any special effects fudging is pretty awesome no matter which way you slice it.
From the Special Edition version of the film, many have considered the extended version of the scene where One Night is in Flatbed moving the rig, singing "Willin'" by Linda Ronstadt with Bud and Hippy joining her just shortly before Lindsey arrives as a pretty awesome.
The military medic revealing he had removed the magazine from Coffey's gun is understatedly awesome, especially the way he later ejects the bullets from it one by one.
After being out of commission for a good chunk of the film due to accidentally getting too much oxygen in his breathing mixture, Jammer wakes from a coma after Coffey seals the rest of the Deepcore crew in the rig's kitchen, knocks out one of the marines, takes his gun and frees the crew while looking like he just woke up from a nap.
Bud: How you feeling, big guy?
Jammer: Okay, Bud. Just figured I was dead back there when I saw that angel comin' for me.
Bud: Oh. Yeah, okay. Yeah, why don't you tell us about it later?
The novelization of the film clears up how Jammer recovered. When the water tentacle controlled by the aliens focuses on him, it recognizes Jammer as the human encountered in the military sub earlier in the story. It extends molecule-thin tentacles into his head to fix the damage done to him by breathing the bad air mixture (which was caused by him freaking out and smacking his respirator on a wall after seeing an alien in the flesh). It's even implied that he regained consciousness soon after and was just waiting for a good time to ambush Schoenick for his weapon.
While it's not an action scene, it takes some real badassery to come up with a plan like "tow me back without any oxygen and hope the icy waters will slow down my metabolism enough that drowning leaves me Only Mostly Dead and I can be saved once we're out."
