Currently Halo MCC is number 89 on Amazon's yearly total best seller list. It is the only game up for preorder on the list meaning on Amazon at least it would currently be the game with the most total preorders.
While Amazon is by no means the biggest retailer for video games I can't imagine that other retailers have such a massive difference in preorders that it is actually number 16 on the preorder charts.
Sadly other sites don't have total preorders for the year but it is also currently
number 6 on gamestop's website
number 1 on bestbuy's website