Sweet! I've actually got an enemy in mind. Circe? I'm seriously not picking her because of my user, and I wasn't actually going to pick her.
Research has revealed that Circe is responsible for the Scylla we know today—the big, bad, and undefeatable monster of the seas?—in a fit of jealous rage (Well, undefeatable save for Heracles). Scylla was once a beautiful nymph that got a little too cocky. She rejected a man because of his fish tail, and he went to Circe for help—who, as it turns out, was in love with the man. Jealous of Scylla, Circe cursed the nymph's watering hole and turned her into the monster that we know now.
I figured, if you're capable of creating a monster such as Scylla out of jealousy, then you've got some major potential for world domination. It's also said that Circe is the most powerful sorceress, so there's that detail as well.
Sorry for slight info dump, i just found this really cool and wanted to explain why I thought Circe would be a good enemy.