My favorite is 'Mister Magoo's Christmas Carol'. I know, it sounds horrible, doesn't it. But it's a musical and the songs are all lovely, with an 'Englishness' to them. I watched it when it was first aired in 1962; I was 9. I remember sitting at my friend Gerard's kitchen table with him and my little sister.
Running a close 2nd is the usual versions of Christmas Carol. Then 'A Christmas Story'.
Somebody's blog listed 'The Bishop's Wife' as their number one pick, and I notice several people here have mentioned it. I'll have to check it out.