No part of the world does horror with as much brilliant insanity and bold tenacity as Asia. It's just one of those undisputable facts of life.
To put it bluntly, Asian horror does not f**k around. Very few times will you ever find a horror film from Asia that pulls its punches. This a major reason why most Western remakes of them fail. They're afraid of being too bleak or too disturbing, and feel as though they have to hold back. Either that, or they try to copy the style and themes of whatever film they're remaking without understanding what makes the original so special to start.
If you want terror that will dig itself into the marrow of your bones and make you its home, Asia is the place to go. Luckily, you don't actually have to spend the time and money to go there, as we have access to movies from all around the world at our fingertips.
A word of warning though before getting to the list: some of the movies mentioned deal with subject matter that can and will be triggering to some people. Viewer discretion is advised.