Fear factor wise, i think The Grudge is scarier... The ring did have its moments, but The Grudge, both Japanese (titled Ju On) and English had scary scenes (I'm only judging from the trailer of the Grudge and i've seen Ju On), the Ring's Japanese version wasn't all that, but Sadako (or whatever the girl's name was) is freakier in the Jap version.
Please wait for the following movies to come out as remakes and we can have a mega battle...
Dark Water remake (the original Japanese movie was directed by the director of The ring)
Ring 2, directed by the actual Japanese director of the Ring... so the scares will be better.