Dawn of the Dead is so damn awesome. Except when you are about 17 and you bought the director's cut over your friends' house to watch and their dad walks in on the extended sex scene.
I can't believe it but I voted Event Horizon. It just gave me that creepy feeling when i watched it all those years ago. I was also bedridden ill for 2 weeks when I saw it and lost like 20 lbs from not being able to eat due to some tonsil infection.