Randall Bailey is certainly one that comes to my mind... I saw him fight live two times, damn, can that man hurt his opponents badly!!
The hardest "single" puncher or the hardest "accumulation" puncher. I would say Abraham for the first and arguably Wladimir Klitschko for the second. Bailey, Maidana, Briggs are good picks as well.
Heavyweights are supposed to hit harder p4p. On all of the recordings they do on heavy bags, Heavy weights usually hit harder even for their body weight althought people just don't like to rate them.