Unsurprisingly, Medea is the protagonist of Medea. Her relentless pursuit of vengeance against Jason is the central spine of the play. Medea is the perfect protagonist in many ways, because she causes everything to happen. She tricks Creon into giving her another day in Corinth – time enough to hatch her scheme. She manipulates Jason and her sons into taking poisoned gifts to Creon and his daughter. She murders her children.
The character is a great example of the fact that not all protagonists have to be "good guys." Indeed, a character like her would be a villain in a lot of plays. She's a vengeful, child-murderer. However, in dramatic analysis nobility has nothing to do with whether a character is the protagonist or not. The most important factor is who drives the action. In this light, it's hard to deny that Medea emerges at the end of the play as its blood-spattered but undisputed protagonist.