I always consider Christmas music a very personal thing - seems like everyone usually likes what they grew up with. I have a few recommendations:
George Winston - December. The essential Christmas album. More seasonal than Christmas per se. If you've never heard George, you owe it to yourself to get this album - Christmas or not.
Amy Grant - various. Amy has three Christmas albums out - you can't really go wrong with any of them. I think my favorite is "Home for Christmas."
James Taylor - James Taylor at Christmas. This a new one, but it's got a nice acoustic guitar feel to it.
Perry Como - I Wish it Could be Christmas Forever. I grew up listening to Perry Como LPs every Christmas, and I think this is about as close to what I remember as is available. He sings all the classic Christmas songs as only he can.
Jim Brickman - The Gift. Another piano-based item. Jim has his own interpretations of the traditional Christmas songs.
Various - Christmas Songs. Nettwerk records Christmas release. Barenaked Ladies, Tara MAcLean, Dido, Sarah McLachlan, Maren Ord, Kendall Payne, and many others. It's on this list for two songs - Sarah McLachlan's Song for a Winter's Night, and Tara MacLean's Winter Wonderland. But then, I'm biased toward the two.
Various - A Christmas to Remember. Again, worth it for the Tara MacLean track "Light of the Stable." It's one of those songs you've heard before, but no one ever seems to put it on a Christmas album. Unfortunately, this one is out of print, but it can be found on ebay and in used CD stores here and there. Sometimes a place like Borders will have it in the discount Christmas bin.
My favorite Christmas CDs though are a set I put together several years ago, which combines my favorites from about 20 different Christmas CDs. I didn't even mention Suzy Bogguss, Garth Brooks, Martina McBride, and a host of others who have all crafted at least one really great Christmas song over the years.