Based on the equip system I'd say Cloud. The materia system would make him too overpowered.

Mime, KoR+hp/mp absorb, w-summon, phoenix+final attack, you all know the rest. And add with that a ribbon and enough blabla sources to have all stats maxed.

I havent played through FFX then, but What I've heard you can have a crapload of life and damage, so a highly lvled Tidus would beat Zidane and Squall.

Squall would most definitely beat Zidane.

That's based on game system, character wise it's what Tidal Ralique said.


Edit: And I havent had the time to play through FF 2-5