What about Jurassic Park (1993). Though not a pure sci fi horror it can be interpreted as such. When i first watched at the age of 9 it scared the sht out of me. Incidentally i just realise that was the first film that ever scared the shit out of me.
The scariest ones: -The Thing (1982) -The Thing (2011) -Dawn of The Dead (2004) -Fire in The Sky (1993) -Alien (1979) -Aliens (1986) -Prometheus (2012) -The Terminator (1984) -Terminator II: Judgment Day (1991) -Terminator III: Rise of The Machines (2003) -Blade Runner (1982) has this very weird, dark thing which disturbs me. -Predator (1987) -Predator II (1990) -Predators (2010) -Altered States (1980) was weirdly scary. -Videodrome (1983) was so bizarre the it scared me so much. -Event Horizon (1997) was P.W.S. Anderson's best film.