Had to go with:
JACOB'S LADDER - just watched it on New Year's Day and it still holds up! (I still recommend the experimental viewing of this and HELLRAISER as flips sides of the same gnostic coin)
THE EXORCIST - because I'm an atheist and it's still one scary ass movie! The sound design is amazing and Mercedes McCambridge as the voice of Pazuzu is great, great, great - "a vulgar display of power...".
DAWN OF THE DEAD (original) - probably my all time favorite horror movie EVER, or at least in a rotating spot for top 3, this was my STAR WARS. The remake, while better than expected, doesn't even come close. "You can't have it. We took it. It's ours!"
LET'S SCARE JESSICA TO DEATH - an overlooked gem that used to play all the time on Saturday afternoons of my youth. AS Kim Newman put it, "the love generation meets the living dead". The last image is stirringly poignant.
Addendum - if there is the chance of a mop-up last category for overlookeds, I'd like to nominate MARTIN, which I probably forget to mention in the thread that started this. The most thoroughly modern vampire movie ever made....