Do They Know It's Christmas, Band Aid. Christmas+the 80s+a recording studio full of Brits = the highest form of bliss.
All I Want for Christmas is You, Mariah Carey. I'm not a Mariah fan, but I love this song. If you don't like this version, you probably don't like puppies, rainbows or ponies either.
It's Christmas (Baby Please Come Home), U2. So much angst, Bono, and at Christmas! I hope she finally came home.
I love Tchaikovsky's Nutcracker from start to finish. The Chinese Tea dance is one of my favorites:
Little Drummer Boy (Peace on Earth), Bing Crosby & David Bowie. The dialogue at the beginning is kind of cheesy in that "Paul, I think I told you, I'm a lover, not a fighter" kind of way, but it should be obvious by now that a little cheese will never deter me from liking something.
Hallelujah Chorus from Handel's Messiah. Gives me chills every time. Chills I tell you!
Followed closely by For Unto Us a Child is Born, Handel's Messiah.
Shifting gears, we have Blue Christmas by Elvis. Elvis's Christmas Album was a mainstay of my childhood, and for me it's just not Christmas until Elvis starts crooning (sorry, I tried to find young Elvis but could only find 70s Elvis on You Tube.)
Ah, but here's young Elvis singing Peace in the Valley, which I might even love more than Blue Christmas!
Gabriel's Message, Sting. This is a beautiful song, and I don't think it's ever gotten the play that it deserves.
Ave Maria, Harry Connick Jr. Harry takes his sweet time getting into it, but I love his version.
Do You Hear What I Hear?, Whitney Houston. Oh, Whit. That Bobby Brown really did a number on you.
O Holy Night, Josh Groban.
Carol of the Bells. Amanda introduced me to this version by the a capella group Straight No Chaser:
Christmas in Killarney, Bing Crosby. I pretty much want to live inside this song.
That's a good start. There are more, many more. I'll be adding them as I remember.