I've recently discovered that Antonin Dvorak's "Stabat Mater" and "Requiem" are really awesome, especially for those of us who like strong, powerful performances. They are available on iTunes and Amazon.
On the more mellow side, I also recommend Hearts of Spaces "Sacred Treasures" series, (there are five cd's in the series), the best are the first, fourth, and fifth. They are very relaxing and beautiful. They are also available on iTunes and Amazon.
For those who like getting really deep into Eastern Christian music, there is the series "Orthodox Shrines of the Russian North" on iTunes.