Voldermort. Reason? A high magic concentration in an area disrupts any and all technology used in that area. That's one of the most basic rules of the Harry Potter universe, it's the entire reason why wizards don't use technology. Even watches more advanced than a pocket watch won't work at Hogwarts. Therefore, none of the technology Darth Vader uses, like a lightsaber, will work provided he's facing Voldermort in a place of magic, which is the most likely course due to Voldy's hatred of anything non-magical. That would kill Darth Vader anyway, what with his life support systems in his suit, but even if it didn't, his only recourse then would be his Force powers, which he'd need to be fairly close to Voldermort to use. Voldy could Avada Kedavra him from a distance, thus winning the fight anyway.
There is literally no argument that can defeat this summary, either, and believe me I considered other methods. If anyone wants to argue then fine, but I'm telling you, unless Voldy is playing the Idiot Ball, there is literally no possible way for him to lose.
