Graviton, because they made him a layered, interesting antagonist who is equally opposed to the "good guys" (SHIELD) and the "bad guys" (Quinn, Hydra, Centipede, etc.) because he doesn't think any of them can be trusted with advanced science. I liked how they took an exciting supervillain from the comics and gave him additional depth so that he would fit in perfectly with the world of the MCU. I can't wait to see his return next season with his full gravity-controlling powers.