I wanted to pick Eva as she did achieve her goals and "won" season 6. But we know she's going to lose to Flash at some point in season 7.
While it's cliche, I voted Thawne because even if he lost (again and again) he still wins every time. He caused Barry so much pain such as killing his mom, and indirectly killing his daughter. But everything he has done all started from his desire to become the Flash himself. And even though he's his greatest enemy, the fact that he gets to fight Barry every so often must be satisfying as a Flash fanboy, which we know Thawne is, deep down.