I just finished taping Ju-On: The Grudge (the second Japanese original - the cinematic one, not the crappy made-for-TV one) for some friends at work. I couldn't even bear to watch it again (I would peek in on it with the sound off, and then have to turn away from the horribleness almost immediately). I know of at least two people who consider it the scariest movie they have ever seen.
I'm not sure I would go quite that far, but it is easily the most relentlessly intense and malevolent movie I have ever seen. Most horror movies have clearly defined respites where the protagonists can regroup a little bit and try to figure out what's going on, someplace where you feel reasonably secure that nothing terrible will happen. This movie does not. The evil can strike any place, at any time, and usually in a way that is very creepy and bad and wrong. And all you have to do to bring it on yourself is to visit the Evil Cursed House, or (IIRC) even come in contact with someone who has.
I recommend that everyone who likes to be scared see it exactly once - don't worry too much about the subtitles if you don't like that kind of thing, it's not that heavy on dialogue.
If you can watch it more than once, I applaud you. Seek professional help.