I'm surprised no one has mentioned "A Serbian Film" yet. Sure, it's not a horror film per se, but it's disturbing nonetheless.
REC, REC 2 and the first Descent are fantastic, but you've probably watched them by now.
Even though I'm not a huge fan of Martyrs, I can't recommend it enough. It's one of those movies you just have to watch (like Hostel back in the day), since nothing can prepare you for the mindfuck you'll experience.
Since no one has mentioned this one as well, I'll add Les Sept Jours du Talion to the list. Great concept, mediocre execution, but interesting after all.
And Cabin in the Woods is a comedy, not a horror movie. It's a good movie, just don't believe the hype. It's nowhere near as clever as the filmmakers and the critics think it is.