Roberto Duran in (light) middleweight. I'm not saying he did not hit hard, but only if he had KO power...
Jürgen Blin (Germany) went seven rounds versus Ali. He also was European Champion (won it versus Urtain). Good techniqe, but his punching power was not so great.
Joey Maxim. I will say, in Winky Wright's defense, he had solid consistent pop, he just didn't have one hitter quitter and he was too concerned with not taking risks to stop guys on accumulation. He busted plenty of guys up.
This can often be turned around. Tommy Loughran is well known for starting out as a power puncher, and did all right early on, but the resultant hand problems forced him to convert to a stylist. Without those hand problems, he would not have achieved the greatness he did. Senor Pepe's post #3 gave some excellent answers, and I think I'll add Jimmy Young to former victim Marvin Stinson as a drop out from the Philly school of power punching.
.....i was going to list joey maxim and billy graham, but they've already been mentioned. maxim had a great chin, excellent boxing skills and was strong (something that is usually overlooked). if he had a punch he'd belong with the murderers row group. as it was, he still had a successful career. in an interview he was asked if he ever had damaged hands he laughed and said, "no, i didn't punch hard enough. jake lamotta seldom showed knock out power. if he would have been a greater all-time great.....as it is i still think he was an atg.
I think of Randy Shields...if he had a punch,...any kind of punch, he would have stopped Pipino Cuevas..decked, and stopped him.
I know he's no great or anything, but what about that Cesaer Rene Cuenca (i think thats his name anyway!) who's 42-0 with 1 KO. 1 KO!! Talk about zero power