Do you think that Medea and Jason show arete/excellence or virtue in their quest for the Golden Fleece?

There is no virtue in their quest. Medea uses her powers of witchcraft to help Jason get the Fleece and then performs some despicable acts to her family so that they can get away. Jason, in his "greed" to comlete this quest, is willing to do and say anything to seduce Medea into helping him on this quest. her love for him allows her to act badly; his horrible, materialistic personality is shown once he gets to Corinth and decides to "dump" her and their children.