Who is the winner of the wager? Faust or Mephisto?

I would say that Mephistopheles wins the wager. Faust is until his death condemned to a personal hell as the result of his wager with the Devil. He has experienced grace through Gretchen’s love, and he knows that it exists, yet he is condemned never to experience it again. He will live with the knowledge that he caused the death of his love and that he will never again have his blissful moment of union with Nature and the world.