Oh! I have seen Ringu 2 & 0 recently...:D
Ringu = B-
Ringu 2 = C-
Ringu 0: Bāsudei = B
The Ring (American Remake) = B+
The Ring 2 (American Remake) = C
As far I know there may be another 2 films belong to this franchise:
Rasen: The Ring Spiral (1998) = C-
The Ring Virus (1999) = N/A