User blog comment:Matt Hadick/Vote in Wikia's Epic Dragon Battle/@comment-5014364-20131205004242

George R.R. Martin has actually said, when asked, that Tolkien's dragons would beat his:  fundamentally, Tolkien's dragons can talk and are sapient:  Martin's dragons like Balerion are just animals. So Tolkien's dragons have the edge.

Greatest dragon ever? Probably Ancalagon the Black.