Who would win in a fight?

Harry Potter. While Frodo might have the One Ring, Harry could simply use Expelliarmus. I ume they'd be fighting at the top of Mount Doom, so Harry's spell would cause the Ring to fly into the magical lava, destroying it forever. At that point, either Frodo would surrender or (if he's having another one of his teenage angst moments) Harry would Avada Kedavra him.