No. 6: Tyrannus

Credit: Marvel
First appearing in The Incredible Hulk #5 in January 1963, Romulus Augustulus was the last emperor of Rome before the fall of the empire. Exiled to the cavernous depths of Subterranea, it was there that he discovered the well feeding the Fountain of Youth. Drinking the water Tyrannus became immortal. Also in Subterranea he discovered the advanced technologies of the Deviants. With dreams of creating the ultimate empire on Earth, Tyrannus has returned to the surface time and time again attempting to conquer the world.
One of the Hulk's earliest enemies, Tyrannus has battled against the Hulk and other Marvel heroes again and again. Intelligent, immortal, and a lust for conquest only a Roman could possess. Tyrannus once took over the golden city of El Dorado, and using the technologies therein became a being of pure energy. Tyrannus at one time found himself merged with Hulk's other enemy the Abomination. Perhaps the most devastating blow the tyrant has ever dealt to the Hulk was the seduction of Betty Ross. Tyrannus makes the list for attacking Hulk on multiple levels over and over again.