Having beaten the game a couple of times, I would definitely say Auron. I made him an absolute beast and it was near impossible to bring him down, so he was extremely useful. Even though I couldn't stand Tidus, he was arguably even more useful for me, and pretty much every other character paled in comparison, except for maybe Yuna because of her summons, but I needed them less and less as the game progressed. Until about the halfway point, I would have definitely said Lulu, in usefulness and character, but she became obsolete by the game's end.