I'm not saying it is the worst - there is such a lot of competition, after all - but it is the one I hate the most. Reasons?
1. The basic premise of the lyrics is not just banal, but actively wrong ("I wish it could be Christmas every day", "when the snowman brings the snow"! - and "Now the frosty paws appear/ And they've frozen up my ear" is shameless drivel)
2. Nasty sickly tune, with grinding modulations
3. Appalling muddy musical arrangement and production - gives me a headache just from the texture.