Initially I thought this would be an easy answer, but then I start thinking...
First thought was Perry Como Sings Merry Christmas Music. That was the record my parents would cue up every Christmas morning right before we ran downstairs to open presents. When we heard the first line of Perry's recitation of 'Twas the Night Before Christmas, we could come see the treasures Santa brought. "Sings..." is probably the only album, Christmas or otherwise, that my family has owned in 33 1/3, cassette, CD, and digital formats.
But then I thought about Christmas Portrait by the Carpenters. This album has been the "go to" Christmas album for my wife and our family. I love the way Richard Carpenter mixed this album to essentially be one continuous piece of music rather than a bunch of separate tracks.
I guess I'd flip a coin between those two albums if I had to pick one.