I think the alien in Alien was pretty scary. Along with the slow, uninterrupted build-up of tension.

Nobody seems to agree with me on this, but my sis and I saw Cloverfield in an almost empty theater (an exaggerated imitation of the old movie palaces) on a Superbowl Sunday. And man, did it ever work! I still love the scene in NYC, as hapless partiers are trying to find out what's making the big noises by rubbernecking on the balconies, and the head of the Statue of Liberty slams into the street.