nobody here besides the thing has actually used a full power punch iirc, thing uses his sunday punch on some of the guys here, most notably the hulk so... basically no real measure here if you want the top guys.
so you know what lets make it the strongest punchers by feats lets judge them by there punching feats and also lets add full confidance gladiator into this
Being really strong and being a good puncher are two different things, which is why I brought up Rogue. Superman might have a poweful blow, but that doesnt make him a good puncher. On that list, Namor and Thing both strike me as the best punchers. Thing trained up in boxing, and Namor has always had a impressive haymaker. They might not have the hardest punch, but they are probably the best strikers on that list. If Wonder Woman were included, I'd say she trumps them.
Superman and SMP especially are just sluggers. They rely on strength more than technique. Iron Fist is nasty, but he is asking for guys in the class 100+ range, who generally dont rely much on skill. Thats why I like Wonder Woman, shes obscenely strong but has ability to go along with it, which is rare in that class.
Pound for pound Iron Fist. In terms of pure strength. Believe it or not I'd say Blue Marvel. He had the WTF punch of the year putting Sentry into orbit with a single punch.