Blair Witch Project - it's a bit of a mainstream recommendation these days but it was good when I watched it, I hope it aged well. Paranoia of the cast gets to you rather than relying on jump scares which I think would let it age better than most. It's also probably the only handheld camera film I watched without wanting to vomit from motion sickness.
I've heard good things about the movie "Rec" but haven't watched it yet myself.