I'm partial to Yuna's Nirvana, really.

That's the key to Yuna getting any use. I mean, your average ffx player will go through the game using wakka tidus and auron as much as possible, but when you find yourself facing the fact that YUNA is the only one with a weapon that can break the damage limit, and those suckers in the monster arena are just about OWNING you, you realize that it might be a good idea to level her up some, just so Auron will have someone to fight with, when he gets his masamune.