O Holy Night is always popular. I use the McDonald/Wood arrangement. It Came Upon a Midnight Clear is also nice from those books (I like it because one verse is about “harps of gold”). I also love their arrangement of Greensleeves. I bought this set of carols last year on harpcolumnmusic and it has a nice greensleeves as well as other great carol arrangments: http://harpcolumnmusic.com/all-music/artists/anne-sullivan/english-and-german-carols-set/
I like to use selections from the Campbern Christmas Collection. especially O Come, O Come Emanuel, Noel Provencal by Grandjany, and, new this year (for me), Mary, Did You Know, arr. Erin Freund.