Maybe Kudryashov. There aren't a lot of great punchers in cruiserweight at the moment. If you can really punch at that weight, you might as well roll the dice and try your luck at heavyweight to make some big money. Beterbiev and Kovalev might hit as hard as guys like Lebedev, Drozd, or Glowacki in spite of the twenty-five pound difference in weight.
Once again punching hard and being able to land hard punches are 2 different things. Chakhiev probably hits harder than everyone else but he punches his opponent's guards, not his face.
Chakhiev is a nuclear puncher, regardless of his many failings. Remember the beating he laid out on the very durable and tricky survivor Zack Page? Brutal body assault...
At 175 and proven, Stevenson has the biggest 1 punch, Kovalev puts his punches together better, and Beterbiev might be the hardest hitting of them all.