It wasn't supposed to be about game mechanics, though.
FFVII: Sephiroth vs Weapons. Before he falls into the lifestream? The weapons. After he becomes some demi-god like being in the lifestream? Then Sephiroth could probably take them down. I'd assume, though they differ a lot battle-wise, that all the weapons are supposed to be about equally strong story-wise. And both Sapphire and Diamond were defeated with massive cannons. If human-made cannons could kill them, then Sephy in his uber-mode from thing could probably beat them as well.
FFVIII: Omega Weapon vs Ultimecia= Omega. Ultima Weapon is actually MENTIONED to be "the strongest, ultimate monster said to be impossible to defeat" when you scan him. That should make him story-wise stronger than Griever and debatably even on par with Ultimecia herself. When you scan Omega, it says "even stronger than Ultima Weapon". Also, all the bosses inside Ultimecia's castle EXCEPT Omega are her servants. If it wasn't because Omega's actually stronger than she is, she could probably have made Omega her servant as well.
FFIX: Ozma is... far too weird and obscure to compare to anyone or anything really.
FFX: I'd say Sin could take on any creature, really. So as long as Yu-Yevon controlls Sin... yeah.