That's a great list. I don't agree that all of them are too painful to watch twice.
Requiem for a Dream: seen it twice and it's been long enough that I'd watch it again; no problems at all about it being too painful as its intrusive style regularly breaks the illusion (in a good way).
Dancer in the Dark: is pretty devastating but I would watch it again - would be more painful (but still not TOO painful, and less good) without the musical interludes.
Bad Lieutenant: certainly not a "great" film, too OTT and silly to be properly painful, and just not good enough to watch again; though I might anyway before seeing Werner Herzog's version.
Straw Dogs: watched this more than once, it's unpleasant in parts but not really painful.
Audition: watched this twice, probably wouldn't again, is very horrible but not unbearable. I want to show this to someone who's never heard of it.
Irreversible: I did watch this twice in one day, the first time properly by myself and the second with others who I wanted to show it to and gauge reactions - for the horrible parts I sat somewhere I couldn't see the screen. It would take a lot to make me watch it again, but if the chance came to see it in a cinema I'd probably make myself.
Grave of the Fireflies: very very painful, but I expect I'll see it again sometime. Probably my saddest on this list.
Leaving Las Vegas: shouldn't be on the list. Getting drunk isn't sad or painful!
Million Dollar Baby: I wouldn't watch this again because it's boring, not because it's painful.
Nil By Mouth: was an amazing, electrifying experience that I didn't want to end. Certainly no qualms about seeing it again, though I can see why some wouldn't want to as critic Tom Shone of the Sunday Times said that, given the choice, he'd happily unwatch it.
Antichrist belongs on the list but I guess the list was done prior to its release. Possibly
Breaking the Waves too.
Funny Games (old one) was pretty painful but not bad enough to stop me watching it again. I think a lot of people wouldn't want to an intact cut of
Cannibal Holocaust twice, arguable "greatness" of the film notwithstanding.
I've had a
United 93 DVD for years but still don't dare to watch it as I know it'll be extremely painful. I suppose it's unlikely to be great but there's a 1988 Hong Kong film called
Men Behind the Sun which is pretty hard to get hold of that I probably wouldn't ever dare to watch as it depicts ghastly Unit 731 experiments, and reportedly contains actual autopsy footage of a boy, and other things that I don't want to have in my mind.