A Song of Ice and Fire (contemporary writing I assume), it lost to Harry Potter book against one of GRRM’s best books/chance for win in novel category one year and Spin in another year(Accelerando was another nominee that year)
GoT is undoubtedly more complex, interesting world, but it never had a generation of children reading the midnight book release. That the author actually penned the series finale is also worth something.
But there can't be many metrics at all where GoT beats out HP for the title of "biggest fantasy series of our time".