I've gone for Itoje just ahead of Retallick for the world's current best player.
Admittedly my viewpoint will be biased by the fact that I see Itoje far more than Retallick, but I've given Maro the edge because of his athleticism.
Also, whilst I'm on the subject, I would never consider a Back to be the best player in the world. To my mind there are 8 core elements/skills in rugby; passing, tackling, running, kicking, scrummaging, mauling, rucking, and the lineout. The skill levels of most Backs are either poor or non-existent at 4 of these core elements.