Its not completely out of the realm of possibility that this series has seen a premature end, and yet I still would call it the greatest fantasy series of all time.
Martin is standing on the shoulders of giants here certainly, Tolkien especially I think, but his series is mature in ways that others simply cant match.
For some reason the parallel that enters my mind is the television series The Wire, a show that at first glance looks like a normal police procedural but actually goes so much more in depth when it comes to realism that its practically its own genre.
The same holds true for A Song of Ice and Fire, where despite the fantastical setting, people behave like real people with real motivations for their actions.
In a genre where many writers build fantastically deep worlds only to fill them with characters who are mostly cliche, Martin rises above the crowd, and thats why, even if he never does get around to finishing it, we think A Song of Ice and Fire is the greatest fantasy series of all time.