Wilhelm vs. Ganon, Sephiroth and Kain
*raises fists*
I'll take all 3 fanboys on!
Now most people here know the power of Xenosaga's Testaments.... Wilhelm made them.
Second, to use some powerscaling, here's what one Zohar Emulator can do. (it's what makes the planet go boom)
YouTube video
All 12 of those gave power and creates Gnosis. These Gnosis were beaten by the party's E.S.es. Canon states Wilhelm's Joshua, which is made of Wilhelm's power, is a lot stronger than thsoe.
So...Wilhelm > E.S.es > 3 Emulators > 1 Emulator = Planet vaporizer
Hm...what else. Oh, the Testaments shut down Proto Omega's Phase Transfer Cannon which was going to destroy a planet. They can't absorb Wilhelm's power though or they'll be annihilated. He also owned them with a wave of his hand.
And uh...hm. I can't think of much else for now.
So yeah I say he wins.