Probably Nightmare on Elm Street, started a great franchize and has been my most favourite since I can remember.
Everyone will have their own opinions about the BEST horror movie, for instance, Kazenji above me stated American Warewolf in London, and I personally didnt like it.
But the other choices were bang on, shame they dont make them as good as those anymore.
Here are my favs(I still need to see a lot more, but here's so far):
Child's Play
City of the Living Dead/Gates of Hell
Day of the Dead
Evil Dead
Evil Dead 2
The Fly
Friday the 13th
Friday the 13th part 2
Fright Night
Halloween II
Happy Birthday To Me
Hellraiser
Night of the Creeps
Nightmare on Elmstreet (original, and ONLY original)
Poltergeist
Scanners
The Shining