In no particular order:
The Exorcist
Night of the Living Dead
Dawn of the Dead
Day of the Dead
House of a 1000 Corpses
Devil's Rejects
Demons
Phantasm
Halloween
Salem's Lot
Prince of Darkness
The Thing
Texas Chainsaw Massacre
Friday the 13th
Hellraiser
The Hills Have Eyes
Event Horizon
Last House on the Left
Halloween is my all time favorite horror movie. The Exorcist is #2. I love the horror genre in general and it's hard to delineate my reasons why I hold certain movies in a high regard. I have a great fondness for the old-time movies too, such as Frankenstein, Dracula, et al.