I say that, but I'm not going to die on a hill over it. Joy to the World, Silent Night, Once in Royal David's City (though almost impossible for most people to sing), and O Come All Ye Faithful/Adeste Fideles are all excellent. But the Holly ad the Ivy combines sophisticated Christological imagery with a delightful melody and a cheery feel that makes it great, and a perfect bridge between hymn carols and feasting carols.
Two on this list were not originally written in English. But for what are probably complex historical reasons English as a shocking abundance of spectacular Christmas carols. So it hard to choose among them.
If you enjoy Christmas carols, particularly the old English ones that run roast pig and the Nativity all together, find a copy of Carols for Christmas by David Willcocks.
One of my favorites as well - and I've been in that church