The Strangers came out last year, and I've just caught up to it. It's remarkably stripped-down, almost flat: couple terrorized in an isolated house by nameless, silent people. Very rarely do I get genuinely nervous watching a film.
While it wobbles a little bit - the last quarter meandering into commoner tropes - the first three acts unspool beautifully while slowly, and expertly, building dread - the soul of true horror.