There's a Japanese film called Cold Fish which is based on the real story of a creepy rich psychopath and his girlfriend. The two of them are murderous criminals who are so ballsy that, at one point, they even murder a member of the yakuza.
The movie itself is really well done and has a very dark, twisted vibe. The thing is, it's not some dumb gimmicky splatterfest that you'd see from Tokyo Shock or something like that. But the gore is is so abundant and realistic that it rivals any other horror movie or thriller that I can think of. There's no holding back and, with some of the murders, you see the entire thing including the dismemberment and cleanup. It might sound like shockploitation, but it's handled with a sense of realism that makes you watch the killings and the aftermath and really think about what goes into it and how a mind can be warped enough to pull it off.
Not Japanese, but there's a Hong Kong "slasher" called Dream Home that takes the genre to a whole new level. I won't spoil anything, but it's not some masked madman who is doing the killing and there's actually a plot and rationale behind the carnage. The gore is really top notch and brutal, but not cartoonish in the least. There are some innovative, horrific kills that I've never seen done anywhere else. I was a little stunned after seeing it because the effects were done so flawlessly and gruesome, but the killing itself was troubling and really drove home how evil and heartless the act of murder truly is.
As far as favorite directors, I'm a big fan of Takashi Miike and think he's one of the most talented and versatile directors out there. Sometime's he's hit or miss, but it's amazing that the same mind directed The Bird People of China, Gozu, Zebre-Man, Ichi the Killer, The Happiness of the Katukuris and The Great Yokai War. I can't think of anybody who could create such vastly different, but equally enjoyable films. From the beautiful and heart warming to silly fun to action-packed fantasy to sex-filled chaos and violence and puzzling creepiness. The man can do it all.
