i really don't know who's the number one hardest puncher but here are a few who might qualify:

wladimir klitschko
sam peter
tavoris cloud
kelly pavlik
alfredo angulo
manny pacquiao
marcos maidana
brandon rios
michael katsidis
juanma lopez
rafael marquez
wilfredo vazquez jr.
fernando montiel
giovanni segura
roman gonzalez

i'm sure i missed a few but this list isn't very bad i guess. and by the way, the true no.1 p4p puncher is dead. his name was edwin valero.