No. 2: The Abomination

Credit: Marvel Comics
First appearing in Tales to Astonish #90 in April 1967, Emil Blonsky was a Russian operative spying on General Ross' military projects. Discovering a gamma powered machine, Blonsky exposed himself to far more gamma radiation than had created the Hulk. This transformed Blonsky into a hideous monster with twice the strength of the Hulk, but unlike the Hulk Blonsky retained his intelligence but could never revert back to his human form. Blonsky became known as the Abomination, one of the strongest beings on Earth.
In their first encounter the Abomination easily overpowered the Hulk, going so far as to beat the Hulk to death. Kidnapping Betty and leaving, General Ross was forced to revive Hulk through means of defibrillation. Using one of Banner's gamma siphons, General Ross and Hulk were able to significantly reduce Abomination's strength to that of the Hulk's. The Abomination is a long time enemy of the Hulk and, like Wendigo and Xemnu, one of the few beings on the planet who can go toe to toe with the Hulk in sheer brute force. The Abomination was responsible for the death of Betty and has been one of the few enemies to have bested the Hulk on more than one occasion.