Occurence: Inhumans #12

The Inhuman Karnak has the ability to find the weakest point on a person and strike there. While fighting the Hulk, Karnak concentrates and finds the Hulk's "weakest" point, which is in the center of his chest. He punches there with all his superhuman strength and usually this would take out his foe. The Hulk just stands there and doesn't even move, not even affected by the blow. Therefore you can conclude that the Hulk doesn't have a weak point at all.

Hulk's Physical Resistance Feats

Incredible Feats of the Hulk Main Page