Squall - Aura + Lionheart Limit Break makes for a serious beating

Tidus - Squall would beat him till he stoped being such a baby. Once that happened, Tidus is a character who has many chances to attack, and Blitz Ace is a good finisher. Hastaga makes for lots of fun!

Yuna - Anima, Magus sisters and Bahamut. Maybe Squall can junction Eden onto her as well? Maybe not, who knows but 1mp Holy and every other spell makes things really nice on just about everybody.

I would say Zidane, but in a fight against full size people like Squall, he would get his butt kicked, so even though he has wicked swords and stealing I will pass. I thought about Cloud as well, however with Cloud and Squall in the same group it could make for some confrontations, plus I see them as close to the same ability wise.
So my final character will be...Can I take Sephiroth?

Vivi - Once he steal's Lulu's celestial weapon he will be the best mage of all time, though he lacks in eye candy, unlike Lulu. I will take Vivi though, because nobody expects him to be so deadly, plus he has a hat.