Who is the "Top Dog" ?

[Sephiroth]

Squall was holding his own pretty well against the likes of Sephiroth at the beginning of Dissidia, I think he can hold out against Cloud okay.

Dissidia matches do not exist to show canonical powers and their equals. Squall is a skilled SeeD but he does not hold his own against Sephiroth. There are only two main characters that can hold their own against the antagonists when it comes to the antagonists doing more than just playing around a bit and those are Lightning, who has become very powerful and Cloud with his concept of an "actual limit break" when fights for what he loves. And even then some villains still play around.

X is the only one of these games that can break the 9999 standard I think.

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