So I’m pretty sure the Glee Christmas albums surpass even Mariah Carey and Michael Buble, as far as dedicated Christmas albums go by a single artist/band/ensemble (I’m sure there are compilations that are superior, although none spring to mind immediately.)
A large part of it is because many members of the Glee cast have exceptional vocal talents (Lea Michele was born to belt out some of those classic Christmas carols, seriously). Part of it is because they don’t always stick with the classics—on occasion, they go for fresh arrangements, intriguing mash-ups, or new songs. Part of it is the variety of singers involved, as some Christmas albums by a single artist can get a bit monotonous if not done properly. But I think a large part of it is simply that, because of the nature of the what the television show is, many songs (obviously) sound like a choir is actually singing them. In some cases, this conjures memories of Christmas carolling. In others, a church choir. In either way, the songs are innately Christmas.
I love it.