I started to love horror when I was ten and started reading Edgar Allen Poe. I would sneak out of my room to watch scary shows at night, and by the time I was 11 my mom would let me stay up alone, with my dog at my side, to watch the late night horror shows on weekends.
By FAR the scariest movie I have seen is The Excorsist. Get the re-release which has one scene that actually freaked the HELL out of me many years after having seen the first one.
Also on my top list of favorites: Seven, The Ring, Alien, The Shining (the blood pouring out of the elevators... omg), Pet Cemetary, SAW, Salem's Lot, Nightmare on Elm Street, Carrie, House on Haunted Hill, Blair Witch Project, Hellraiser, JAWS, Sixth Sense, Wolf Creek (truly one of the more horrifying I've ever seen and loosely based on real events)