See,
I thought The Eye was boring an overrated.
One Missed Call was OK but didn't really do anything for the genre (nor did it really try to; it's Miike's version of big budget Onryō).
Tale of Two Sisters was a bit overwrought and overrated and its American remake looks abysmal. But people like it. A lot. I was disappointed.
Shutter was fantastic - Creepy, scary, derivative, but very very good. I ABSOLUTELY recommend if you want a nice ghost shocker.
Also to check out (in Asian):
- Three... Extremes
- Rinne (Reincarnation)
- Du saram-yida (Voices)
French:
- À l'intérieur (Inside) <- HIGHLY recommend
- Ils (Them)
- Frontière(s) (Frontier(s)) <- (I personally wasn't a fan, but I know it appeals to people)
- Haute tension <- (I personally wasn't a fan, but I know it appeals to people)
Also, depending on what you mean by "most frightening" (you DO realize how general that is, don't you?), that could include "gory and fucked up." Check out Ôdishon (Audition).
And I'm with Neverending - Go to the JA forum. Got a LOT of good recs from people in this thread:
http://horror.com/forum/showthread.php?t=32599