Up until a couple years ago, the person who won Best Director almost always won Best Picture, and while not always a guarantee anymore the correlation is still close:
 
BEST DIRECTOR NOMINEES
Tom McCarthy (Spotlight)
Adam McKay (The Big Short)
George Miller (Mad Max: Fury Road)
Alejandro González Iñárritu (The Revenant)
Lenny Abrahamson (Room)

WHO WILL WIN: Alejandro González Iñárritu
WHO SHOULD WIN: George Miller
 
With all due respect to the other nominees in this category, this is a two-man race: George Miller and Alejandro Iñárritu. The other nominated movies are told in a much more straightforward manner than The Revenant and Mad Max.  In terms of getting the most out of his performers, Iñárritu may have a slight edge, as both movies feature Tom Hardy, but Hardy got his own nomination for The Revenant.  However, after Mad Max premiered, Hardy apologized for having doubted George Miller and signed on to play Mad Max again, whereas he hated Iñárritu, and allegedly choked him out on set. Not only that, but Miller didn’t have over 1/3 of his crew quit out of frustration with the working conditions; that should count for something.

In its technical aspects Mad Max is leaps and bounds ahead of The Revenant.  Not only did Miller receive more technical praise from his peers and receive the most guild award nominations, but he was also able to accomplish more on screen. Both directors wanted to shoot their movies in sequence but only Miller did. While there is very little dialogue in both, Miller shows more with less words than Iñárritu, both in terms of themes and visual pop. George Miller took an action movie and made it look so polished that the Academy had to take it seriously, something not even Christopher Nolan’s Dark Knight could do. Both men are deserving nominees for what they accomplished on screen, but George Miller is more deserving.