9. The Thing
The decades long rivalry between The Incredible Hulk and Fantastic Four member Ben Grimm, better known as The Thing, is one of the most well-known rivalries in comics. The pair have done battle many times, usually with an inconclusive victor. However, at least one confrontation left The Thing clearly worse off.
In Fantastic Four #167, Hulk is in the custody of the Four, who are hoping to cure him and restore Bruce Banner. The Thing is guarding Hulk, but helps him escape after the Gamma radiation leaking from Hulk disorients him. Fearing the radiation could kill The Thing, The Fantastic Four pursue the pair. Thing comes to just in time to save Reed Richards from being killed by The Hulk.
Seeing this as a betrayal, The Hulk sends The Thing flying with a brutal punch, just as the radiation exposure reverted him to his human form as Ben Grimm. Ben would have certainly been killed if Fantastic Four teammate Johnny Storm hadn't stepped in to rescue him.
Sadly for Ben, who views being The Thing as a curse, the reversal was not permanent. Even worse, any bond that had formed between the Hulk and The Thing was shattered by the Thing's perceived betrayal, adding to their enmity.