Wow, multiple Choice *votes for all of them*

Seriously though, I'd have to go with Masamune or Nirvana. Masamune looks cool, and Nirvana is really good (does more damage the higher Yuna's MP is I think, plus one MP cost). The problem with a lot of the celestial weapons is that you need to keep the character healthy for it to reach it's potential, except for Masamune which does more damage with less health.