Over the last month I have listened to their first four albums for the first time in over a decade. The first three are phenomenal. Good bit of experimentation although I think some of that was echoing what the Smashing Pumpkins were doing. Still, those three albums stand equal with any three from Nirvana, Pearl Jam, Soundgarden, etc, and their style develops quite a bit from one CD to the next. The fourth one has a good guitar lead in for the first track and does some other nice things, but not nearly as notable. I'd say White Pony was their masterpiece although I could see people choosing any of the first three.
Deftones are one of the few bands from the "nu metal era" that still hold up. I consider White Pony to be a formative album from my teenage years. I tuned out for several years but came back to them with the release of Gore, which is a great album front to back and got caught up on their albums I had missed out on. If you like the more "dreamy" stuff the Deftones put out, I'd recommend checking out the Spotlights, I think they even toured with Deftones a few years back.