Every band or vocalist worth their recording contract eventually releases a Christmas album. Even Bob Dylan has a Christmas album—it’s inevitable. But figuring out which particular songs are the best sellers is pretty tricky, especially with so many covers out there. Even worse, a lot of Christmas songs were written before anyone kept (or had the means to keep) comprehensive sales records. These five songs are without a doubt bestsellers, though—every single one has been covered by Mariah Carey and was probably performed by the cast of "Glee."