I once saw an ad for "The Most Relaxing Classical Music in the Universe," which was faintly ridiculous to me. Yes, I know people who are put to sleep by classical music, but as a whole, I'm no more relaxed by Vivaldi than I am by AC/DC.
If you really need to zone out, though, and you want classical music to do the job, you should probably listen to "Divenire." It's an album by Italian composer Ludovico Einaudi. Here's a short clip from the album, along with an interview:
It's easy to screw up this kind of material. Go too far one way, and you've made elevator muzak that will strain the patience of the listener. Go the other way, and you're aping John Adams instead of forging your own meditative/contemplative atmosphere. Einaudi is classically trained, though, so he manages to avoid most of these pitfalls.