I'm a major pussy and have sworn off all horror movies because they scare the shit out of me and I'll have tons of nightmares and not sleep for days. Seriously I'm like a little girl, its terrible!
But I have seen scary movies in the past and the shit that gets me the most are zombie movies.... I am fucking terrified of anything zombie related, a TV ad with zombies will creep me out at night, Fuck even Michael Jackson's Thriller video scares me lol
That's why probably the scariest movie I've ever seen is "Dawn of the dead" - actually watched that entire movie during a work out, couldn't fucking sleep.
Another one that's not as scary as it is good, 28 days later
Bram Stokers Dracula (with keanu reeves and gary oldman) is sooooo well made, the visuals are great!
Also, for your reference,'I am legend' with Will Smith scared the shit out of me, dunno why but it did!
Honorable mention, not the movie but the original game: Resident Evil, playing that late at night with my cousin with all the lights off and the dogs jumping g through the windows (you had to have played it to know what im Talking about)