Jon Jones
Fedor
Khabib
GSP
Demetrious Johnson
If we are talking about MMA, honorable mentions are Cain Velasquez (to few fights but in his short lived prime, perhaps the best HW of all time), Anderson Silva (one of the best of all time, but fell of hard at the end and sold his soul to the devil when he took a fall for Jake Paul), Junior Dos Santos when he was on his run, 9-0 in the UFC at one point, wins against Werdum, Cro Cop, Shane Carwin, Roy Nelson, Cain, Frank Mir, Mark Hunt, Stipe Miocic, Derrick Lewis, incredible in his prime.
Daniel Cormier's gotta be up there as well, LHW and HW champion, only lost to Jon Jones and Stipe Miocic.