He's in my top 3 because of his delivery of punch, and all aspects together.

But in terms of pure power alone, it's close between a lot of fighters.

Frank Bruno had an absolutely, devastatingly powerful right hand, but nowhere near the speed, agility or technique Tyson had. I remember reading something about a test they did with Bruno on his punch and the power was apparently incredibly high.

Lots of people have power but Tyson knew how to use it.