Uther was certainly the most spiritual paladin, and the first. But he's not one known for being the greatest fighter ever known. In terms of sheer might, Turalyon and Mograine were fighting paladins. They were the ones who'd run into armies like a maelstrom of glowing, righteous death. He's the guy who embodied 'paladin' though, so technically he's the best? That's my argument at least.