The ones I like...
1. The Holly and the Ivy (Christian or Pagan versions)
2. The Sans Day Carol
3. Gaude Te (especially when performed by the Medieval Baebes)
4. O Little Town of Bethlehem (but not the last 2 verses)
5. Stille Nacht (the original version of Silent Night)
Nominations welcome. If you are reading this, consider yourself tagged - blog your own list.