Christmas in Australia by Roaring Jelly and Jogging along on my reindeer by John Kirkpatrick are fum Christmas songs and the albums Christmas Album by Jethro Tull and In the Bleak Midwinter by Gordon Giltrap are certainly worth a listen.
Most hated? I agree with a previous contributor; anything that commercialises the season and, of course, THAT AWFUL record by Slade and the one by Roy Wood's Wizzard.