Alright, to start off, we are not basing this on in-game mechanics this was more of something I always wonder is whether the super bosses of FF lore are actually stronger than the final boss in there respected games from a story standpoint.
Can ExDeath really defeat Omega? Could Sephiroth really take on the WEAPONS and win? Is Ozma stronger than Kuja? Is the Omega Fiend on par with Yu-Yevon? and is the Undying powerful enough to beat the legendary Yiazmat?
Remember, this is from a story standpoint so I don't need mentioning of how Ultima is Kuja's "I win" button or how Sephiroth could use Knights of the Round to beat the WEAPONS. Game mechanics exist simply for the game or to pull off certain elements of the plot but should not be factored in.