There's been a lot of big hitters so to choose just one is kind of impossible.
Too many weight classes and the criteria don't really fit all of them.
For instance...
John Lineker is probably the hardest hitter at 125-135lb weight class.
145 you have Jeremy Stephens, Emmett, Topuria, Conor
155 Justin Gaethje, Poirier, Jens Pulver hit hard, Gomi...hell even McKinney hits like a truck.
170 Woodley, Lawler, Marquardt, Hendricks, Ellenberger ...etc
185, probably Poatan, Drew McFedries, Hendo, Manhoef, Yoel...etc
205, Id have to say Rumble is hard to beat for pure punching power, but Chuck, Wandy, Rampage...etc
Heavyweight, Ngannou, Lewis, Hunto, Fedor, Brad Kohler, JDS, Nelson way too many to name.
These are not in any specific order, but just a couple off the top of my head.