Wolverine Publicity/Oral Tradition

Medusa. Despite the fact that her only solid presence in the original myths is against Perseus, her woobieness in retrospect and being a stock Shout-Out means that if someone's doing anything remotely related to Greek Mythology, she's pretty much an obligatory presence even if claiming to be mythologically accurate yet making the setting during the time of The Argonauts as a reviewer noted in Rise Of The Argonauts that she's just there to be there. Granted she's not the only one in the myths, but she's a perennial favorite to use.
Hercules. Hercules, in some plays and stories, at some point, was in almost every major thing Greece ever did, including riding along with Jason and the Argonauts. He also apparently stopped off at every island and city-state he could find and had about 300 kids, all of whom are the ancestors of kings. He often makes cameos in stories where people go to the underworld, as well. Even accounts of the Trojan War, which took place decades after Hercules' death, have one of his sons make a cameo appearance (and be killed by a son of Zeus despite his own divine heritage).