Chronologically, the order of the films is Ringu 0, the original Ringu, and finally Ringu 2, although Ringu 0 was the last movie to be released. The series could be watched in chronological order (with Ringu 0 first) but, if you don't want the mystery of the first two movies spoiled, then watch the prequel last.
I prefer watching it first, but if you want more mystery in the other films, watch them in their release order: Ringu, Ringu 2, then Ringu 0. Note that there is also a Japanese film named Rasen (aka Spiral, 1998) which was meant to be a "Ring 2", but that was later superseded with the 1999 Ringu 2.
