Such cynicism in one so young. Only a fool would dismiss Christmas (Baby Please Come Home) by Darlene Love or Just Like Christmas by Low. I happily play them in July. Fairytale of New York is undoubtedly inspired but it's all played out with me. If I didn't hear it again for ten years I'd maybe reach for it at some point.
Other than those, Lennon, Slade and Wizzard surely came up with three perennial classics (I know someome who actually prefers Macca's Wonderful Christmas Time to Lennon's song, the weirdo). Run DMC's Christmas in Hollis is pretty ace too. Never quite understood the fuss about The Waitresses' Christmas Wrapping, pleasant thought it is.