In which I reveal my slightly embarrassing fondness for neo-Romantic treacle.
Morton Lauridsen’s Lux Aeterna–not entirely a Christmas piece, but the first movement, O Nata Lux, (text, and several medieval/Renaissance settings here) certainly qualifies. The fourth movement, Veni Sancte Spiritus (an ok performance, helped a good deal by a resonant performance space), is my favorite (text, other settings, notably Victoria, Schütz, here). This recording of the whole piece has to be considered definitive, I think.
And, as Seraphine noted yesterday, Lauridsen’s setting of O Magnum Mysterium is also lovely. (This may be as good a place as any to note that if you live in SLC and haven’t yet seen Brady Allred conduct, you should, ASAP. Also, the world needs more trombone choirs).