Ringu scared the fuck out of me, but I watch horror so little, and am so suggestible to it that it's hard to rank the scare-factor (I did think it was scarier than the American remake, but the Japanese original benefited both from my having seen it first and also just generally being a little more restrained, which usually helps with the scary). In the past few years, my reading has been very skewed towards manga, and I've read a fair number of horror manga that upset me so much I actually had to go lie down for awhile and tuck my feet under the blanket. (Check out Ito's Museum of Terror: Tomie. It's utterly terrifying--also, virulently misogynistic, which is only slightly alleviated by the fact that all the men are pretty loathsome as well, and that's deliberate--and artistically interesting to me as horror that works by being totally weird and over the top and unrelentingly icky. I thought it was much scarier than Uzumaki, and Uzumaki did scare me.) I've also read classic horror manga (a lot of Umezu) that I found at most mildly shocking, although again, sort of artistically interesting, and had fun trying to pinpoint precisely what audience, what age, what mindset would react to this with the proper sense of fear that makes horror successful.