I think Rodrigo Valdez hit just about as hard as any middleweight ever but is KO percentage wasn't great.
Finally, an excellent question! Sorry I'm not more original, but I can't think of anyone better than Chris Eubank.
Dirrell? He can't punch his way out of a milk carton. He only managed to cut up Abraham because he hit him a zillion bajillion times (according to CompuBox).
Several posters already mentioned Jermain Taylor, but that's the first name that popped into my head.
I always heard that 40's and 50's lightweight Ike Williams was a punishing puncher with a low KO percentage.
About Jermain Taylor: Jermain knocked people out left and right on his way up. The dramatic drop in his KO ratio had a lot to do with his competition. Hopkins, Wright, Pavlik, Froch, Abraham! No one ever stopped any of those guys. Lacy has only been stopped once when he quit on his stool in his last fight.