Who is the "Top Dog" ?

[Sephiroth]

Oh sure if you want to go that way nobody can take on Tidus. Then again, if you do go that way, you can also say that a truly maxed out Cloud probably has multiple Sneak-Attack + Knights of the Round combos in his equipment and even a single KotR would already knock out Tidus.

There's also the fact that Squall is the only one who can make himself Invincible, by using a Holy War.

Nah, there's not much point in going that way.

Edit: I still maintain that Lion Heart > Omnislash.

As mentioned:

Lightning can deal 999999 damage. But that is not important for story strength.