I feel like I have to separate this off into two groups. Pre-Scream and post-Scream. Scream really kicked off an era of self-aware horror movies, and it had a profound effect on the genre as a whole. Horror's my favorite genre, so I've seen a ton of horror flicks, and am no doubt leaving some great ones of the list.
Pre-Scream:
The Texas Chainsaw Massacre
Halloween
The Exorcist
Blue Velvet (falls in between genres, but it has horror elements)
Kwaiden
Suspiria
The Innocents
Dawn of the Dead
Repulsion
Evil Dead
Videodrome
The Fog
Maniac
The New York Ripper
Post-Scream:
American Psycho
Cabin Fever
REC
Zodiac
Inside
The Descent
Wolf Creek
The Devil's Rejects
Pulse (Kairo)
Audition
Sorry for the long list, but I could have gone on for days listing horror movies I love.
EDIT: To answer the question my top 5 would be Texas Chainsaw Massacre, Halloween, The Exorcist, Videodrome, and Repulsion.