I have to say his beheading of Medusa. (Of course, Clash of the Titans is about pretty much the same thing). Not only does the myth include multiple stages of the quest, it has everything a good movie needs: action, a driven hero, and even a romance. First, Hermes and Athena lead Perseus to the Graeae, who were old women with only one tooth and one eye between the three of them, which they shared; they were also the sisters of the Gorgons (ie. Medusa), so they didn't want to help Perseus find them. Perseus took those from them to force them to tell him where the Nymphs were (they held the items he needed in order to kill Medusa). The Nymphs gave Perseus winged sandals, a special shoulder bag, and Hades' helmet to make him invisible. After Perseus beheaded Medusa with a sickle given to him by Hermes and Athena's assistance with a shield-mirror, two creatures were born from her neck: Pegasus and a giant named Chrysaor. Perseus then headed back home to give Medusa's head to the king, as he had promised. On his way back, he met Andromeda in Ethiopia, fell in love with her, slew a monster, and then married her. The myth ends with Perseus turning the king to whom he had promised Medusa's head into stone because the king had raped his mother, Danae, and forced her to flee into a temple.
It is truly a fascinating myth, with many different aspects that would make a good movie (which is exactly what Clash of the Titans did).