I love these sort of questions. There hasn't been, to my knowledge a study of the hardest hitting MW's but if you want some idea of how MW's compare to HW's then have a look at this study I link to. To surmise it basically says that there isn't much difference at all in speed of punch, so that doesn't determine the difference in power very much. What matters was what they termed 'effective punch mass'. As you would guess a HW has a higher effective punch mass than a MW. The range of effective punch mass is only measured in mere kilos but quite clearly you could work out that the boxers with better technique were the ones who would be able to generate a higher effective mass by getting more of their body into their punch. HW's have more bodyweight so could get a higher proportion of that weight into the punch, leading to a higher effective mass. Anyway here's the study:
http://bjsm.bmj.com/content/39/10/710
Also relating to your question I'm pretty sure that the Olympic boxing team did a study on punch power on it's fighters at the 2000 games and found that Jeff Lacy was the most powerful puncher not the HW's.