Ringu
Classic, not much more needs to be said.
Spiral
It has a few scattered moments of quality (it nails the ending on the beach, for example), but overall it's a bad sequel to Ringu, a horrid adaptation of a very good book, and frankly dull and uninteresting as a movie.
Ringu 2
This was a replacement sequel after Spiral didn't go over so well, but it really isn't any better. It's basically just a string of nonsensical scenes and set pieces, with not much of a plot tying things together. I still have no clue what to make of the weird pseudo-science shit with the water, or whatever the hell happened in that ending.
Ringu 0
Surprise surprise, this is a pretty strong prequel, that deepens the character of Sadako and adds a whole new dimension to the story.