Worst: 'Christian children all must be / Mild, obedient, good as he' in Once in Royal David's City. Vomit city, more like. And I say that finding the rest of that carol incredibly touching.
All of 'Away in a Manger', for similar reasons.
All of 'I Saw Three Ships', which I find completely pointless.
'For known a blesséd mother thou shalt be' is a syntactic horror show from 'The Angel Gabriel' that my toes curl does make.
Best: 'Sacred infant, all-divine / What a tender love was thine / Thus to come from highest bliss / Down to such a world as this' from See Amid the Winter's Snow. Chokes me up every time.
I like carols that set out the basis of Christmas, so 'This is the Truth Sent from Above' and 'Adam Lay Ybounden' are firm favourites.
The whole of verse three of 'O Little Town of Bethlehem' - 'How silently, how silently the wondrous gift is given' - so beautiful.
The final verse of 'Hark, the Herald' is magnificent.