Ketchel or Fitz. Ketchel probably had more brute power, Fitz was probably the more precise puncher and probably delivered his power better.
graziano and jackson are two you mention that spring to mind quick. stanley ketchel...for dropping johnson..and knocking out much bigger men.. bob fitzsimmons...he had a heavyweights punching power.....aparently he knocked out seven heavyweights in one night.
James Toney recently claimed that, out of all his opponents, Merqui Sosa hit him the hardest, and that was at MW. Toney has fought from MW up to HW. He actually said that he believes that Sam Peter has more power than Sosa, but that Sosa still hit him the hardest and had him badly hurt. I'm not saying that Sosa is one of the hardest hitting middleweights ever, but he must have hit like a mule.
I'll always be partial to the ratcatcher. the great BAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAD Bennie:hat maybe not the hardest ever, (who can say), but Briscoe ****ed the world of a lot of guys. Saw him toast Gutierrez in '71 after being down twice in what to this day is one of my favorite fights ever. Kitten Hayward and that Aussie probably STILL see Bennie in their dreams.
Probably not the hardest hitting but three that should be mentioned are Freddie Steele, Rodrigo Valdez and Bennie Briscoe.