There are way to many great gorror films to pop into a top five, so I'll list some honourable mentions also.
1. The Shining
2. Alien
3. Saw
4. The Exorcist
5. The Texas Chainsaw Massacre
Honourable mentions include The Evil Dead, The Thing, Saw II, Saw III, Halloween, Friday 13th, A Nightmare On Elm Street, 28 Days Later, 28 Weeks Later, Dawn of the Dead, The Grudge, Ringu, The Ring and Psycho.