Spoilers lie ahead, so read at your own risk!
I think what makes the climax of REC so effective is how it combines so many scary elements into one scene. The two protagonists, the only survivors, have one last option: the mysterious penthouse on the top floor. They are chased by the relentless infected up the spiral staircase and, upon entering the penthouse, are met with a bizarre display. Religious artifacts and lab equipment everywhere. No one in sight. Then, something escapes from the attic. The lights go out. Now they are alone with a tall, lanky, naked, impossibly thin humanoid figure, blindly swinging a hammer in the darkness. We never get a great look at it, which only makes it more frightening and uncanny. The camera man is bludgeoned to death and the reporter is dragged off into the shadows, and the film ends.Everything about the scenario adds up to one complete, enveloping nightmare.
Imagine it. There is no one coming to rescue you; you can only hope to buy some time before your terrible end. You are plunged in darkness; you can only see through the limited night vision lens on a camera. You're trapped in a claustrophobic apartment where horrors beyond your comprehension have occurred. All you can do is hide...hide and wait for your inevitable demise at the hands of a repulsive, unnatural thing. The monster resembles a seven-foot stick bug covered in pale, decaying flesh. It's absolutely alarming. This scene truly taps into that ancient, universal fear of things that appear somewhat human yet different - the familiar conflicting with the unfamiliar, creating an uneasy effect. Certainly, REC isn't the only film to depict this fear, but I can't think of one that does it better. Terrifying monster aside, the entire sequence oozes a thick sense of dread, discomfort, and grim hopelessness. It's positively suffocating! This is a film that wraps its hands around your neck early on and gradually applies more pressure without letting up. By the final scene, you are begging for any small semblance of relief - relief that only comes when the credits start to roll. I first watched this movie one early afternoon and it still frightened me to the core. It has the scariest movie moment in my eyes. Side note: Javier Botet absolutely killed it (no pun intended) in his portrayal of the infected "Medeiros girl" in this scene. He's a fantastic character actor and has my utmost respect for scaring the hell out of me!