That's not as simple of a question as some on here are making it out to be. The question was, "pound for pound, who has the better signature punch?". That's impossible to answer right now. Conor has 24 fights(21 wins, 18 ko's) and an 86% knockout ratio. Hendo retired with 47 fights(32 wins, 16 ko's) and a 50% knockout ratio. Conor already has more knockouts and only has half the amount of fights. That's a difficult statistic to ignore...BUT...
Conor hasn't had that many fights against world beaters. Hendo's career is like looking at a who's who list of mma legends. HOWEVER, in the few fights against world beaters he's had, Conor has shined and looked nothing short of spectacular. Some might point to the Diaz fights to discredit Conor, but he sat Diaz on his ass multiple times and Diaz is very durable. He doesn't get hurt or knocked down very much.
Also, Hendo didn't turn into the head-hunting ko machine we all know and love until later in his career. There was a time most of you probably don't remember where he was on the boring side, and not many looked forward to his fights.
All things considered, I have to say Conor. He doesn't have that one-punch crunch of Hendo, but he's faster and more accurate with his punch. His technical skill is far superior to Hendo's, and that is what put him over for me, combined with the fact that he has more ko's than Hendo in only half the fights.
Time may prove me wrong, but that's my opinion.