I really like "God Rest Ye Merry Gentlemen," one of the few minor-key Christmas carols. ("O Come O Come Emmanual" is also minor, and a lovely tune, but strictly speaking it's an Advent hymn rather than a celebration of Christmas.)

I'm also very partial to "Deck the Halls," the only familar/classic carol I can think of that includes neither religious references nor any modern-secular stuff about Santa, etc. When I sing it, I feel like I am indeed "trolling" the one and only surviving "ancient Yuletide carol."

Among the religious standards, "Oh Holy Night," "Joy to the World," and "Hark the Herald Angels Sing" are among my favorites, and I, too, miss the good old Latin "Adeste Fidelis."

As a choral performance -- not something I'd try to sing myself -- there's nothing quite like "Carol of the Bells."