Assuming UFC ... Definitely Hunt. Ngannou deserves a mention for sheer power.
But by that logic the list would just be all heavyweights. If we adjust for power/weight ratio, Prime Johnny Hendricks was a beast. Rumble Johnson wasn't as powerful as heavyweights, but LHW Rumble had serious power. Yoel Romero also deserves an honorable mention. At lightweight you had Conor, and right now 135 Petr Yan is a pretty beast power puncher when he sits down on one.
And of course, we can't leave Dan Henderson out of that list. When you get a punch nicknamed the H-Bomb, you've got some power.