Not sure why this was bumped, I still think Stevenson hits harder with his straight left in terms of a single hardest punch, Kovalev is a harder puncher overall. For me it's unchanged.
Both hit insanely hard. I'd give the edge to Stevenson in terms of one shot power. But Stevenson's style is to look to load up and launch hard single KO shot's. He has the wide stance to gain maximum leverage and he has that explosive speed to help him deliver those hard single shots so effectively. Kovalev has a more methodical approach. He has a narrower stance, he doesn't set his feet so wide apart to punch, he isn't looking to land hard single shots, he's just looking to land. They way he fights means he doesn't load up like Stevenson does, so hit shots probably don't have the same force behind them. But the ease with which he hurts people, with either hand as well, suggests he has a ton of power as well. If he loaded up like Stevenson he might hit even harder, but he doesn't have the speed to get away with such a style.