Adeste Fideles... is my favorite carol, I just like the mysterious Latin:

Adeste fideles
Laeti triumphantes
Venite, venite in Bethlehem
Natum videte
Regem Angelorum

second favorite is O Holy Night, it's just so achingly beautiful, I get shivery little goose bumps when the music swells to the chorus:

Fall on your knees, O hear the angel voices!
O night divine, O night when Christ was born!
O night, O holy night, O night divine!