First off, you have a very nice little collection there. Great way to start off! Anyway, here's more suggestions:

-Shadow of a Doubt
-Night of the Living Dead
-Dawn of the Dead (original)
-Dawn of the Dead (remake)
-Day of the Dead
-Land of the Dead
-Evil Dead 2
-Army of Darkness
-A Nightmare on Elm Street
-The Exorcist
-The Omen (original)
-I Spit on your Grave
-Seven

The last five were from personal opinion (them being my top four horror movies, not in order though).

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dellamorte dellamore
evil dead
suspiria
the shining
dawn of the dead
the omen
texas chainsaw massacre
audition
black christmas
sleepaway camp (well a personal favorite and the only slasher that deserves a spot imo)