I have to second A Clockwork Orange and American Beauty.
One vote for Citizen Kane. It was a brilliant movie, it just wasn’t a repeat experience.
How about The Unforgiven as a truly great movie that I have no desire to see again?
Re: “The Usual Suspects”. If there was ever a movie that I HAD to see at least a second time, it is this movie. There are two things to be looking for (and I hope to state these so as not to need spoilers): 1) how much of Verbal’s story can be verified with other evidence (that is, how much of his story really happened). And how much of his story could have been completely made up ? 2) Given where Verbal gets his “clues” for his story, do the logisitics of how he acquired those clues fit in with when these clues appear (more of a test-the-editting game) ?
As to the OP, I would second:
Requiem for a Dream
Mystic River
But disagree not only on “Usual Suspects”, but also “Love is Beautiful” , “Unforgiven” and “Saving Private Ryan” (they’re painful, but worth seeing for other aspects).
I could never watch the movie Iris again - incredible movie, but very, very, very sad.
It’s more than gore that holds me back. Gore really has nothing to do with me not watching Bad Lieutenant another time, I can’t think that there’s much gore in it. Or in Blue Velvet either.
It’s just, I don’t know, emotionally painful. I suspect Trainspotting would be the same.
Disagree on Life Is Beautiful; I watched it again and I think it was even more poignant the second time around, for the exact reason listed in the spoiler.
I couldn’t possibly watch Passion of the Christ again. I’m not sensitive to violence or blood, etc, and I’m an atheist, but that movie was just too difficult for me to watch. I spent most of the film staring at the left ear of the person in the row ahead of me.
No one has said Dancer in the Dark?!
Beautiful-looking film, but cruel almost to the point of parody.
Mention of Iris, with which I totally agree, reminds me of it’s contemporary In The Bedroom which was a fine movie with great performances and I will never watch it again.
Second vote for Threads.
Important film, I thought. Definitely worth seeing. I bought a copy from eBay, in fact, because it’s pretty hard to find. Then I loaned it to someone and never asked for it back.
After watching it, I remember thinking I might put on Requiem for a Dream so I could have a few laughs… 
Super Dave
Python 1 & 2
Duel
and i know i’m going to have to add Saw to the list when i see it in a few days.
It was the first movie I thought of when I saw the thread. As others have, I’ll say the same for von Trier’s more recent Dogville–not quite the downer that DITD was, but the stark set and the deliberately flat performances that were supposed to bring the plot into sharp relief instead made me feel like I had spent a month watching it.
I felt the same way. It didn’t really bug me because it was practically like watching a cartoon. But the scene at the very beginning where Uma Thurman’s character got shot in the head – that was more realistic, and it freaked me out enough that I just barely got around to watching the second part two days ago.
My wife just forced me to watch Thirteen because I made her watch Hell Boy and Bubba Ho-Tep. I by far got the short end of that deal. Man, marriage sucks sometimes.