I would say Kovalev hits harder overall. Their common opponent, Boone, said Kovalev hits harder. Plus, Stevenson's power is in his left. Kovalev can knock you out with either hand.
In regards to Donaire, I would have thought he'd be up there too, but if you check his KO% he's gone from being a great puncher to just a good puncher. Him and Pacquiao both dropped out of the 80% range down to the 60% range. Even though they are still superior p4p boxers with great skills, I don't think they are world beaters with their punching power at their higher weights.