The painful story of Festen was powerful enough to have me label it a masterpiece. Unfortunately that honor didn't survive a revisit. I still consider it very well-made (only a master-director can make such a movie, given its self-imposed limits), and the emotions it stirs are still high-impact, and the story is important enough that I'd promote it heavily if I had the means.
[sidenote] The very first scene is intensely horrid, one where super-asshole chases wife and kids outta the car. It was hard to swallow.