I'm not sure it's the "best," but the book looks pretty good. Black Feathers by Joseph D'Lacey
Black Feathers is a modern fantasy set in two epochs: the Black Dawn, a time of environmental apocalypse, and generations into the future in its aftermath, the Bright Day.
In each era, a child undertakes a perilous journey to find a dark messiah known as The Crowman. In their hands lies the fate of the planet as they attempt to discover whether The Crowman is our saviour… or the final incarnation of evil.