I hate to say it but Adonis Stevenson. Next year though he will be exposed and it may be before he fights KOavlev.
Yes, he won four fights by stoppage, against decent opposition. FOTY is either he or Kovalev. Sadly though, I think he'll avoid Kovalev as long as possible.
Zab's just as talented and physically capable as he's always been. His issues are all mental. He just doesn't throw punches.
Akiria Yaegashi needs to be on that list. He may not warrant the top honor, but he deserves to be considered, given the year he had.