Heyo... i'm thinking I'd Probably go Kain(ff4), Cloud(as much as I hate him), Magus(CT), and Rosa(ff4 again)
You need Rosa cause lets face it, white magic was WAY too strong in ff4.
You need magus cause he can basically cast any kind of spell, and as far as mages go his physical attack is pretty decent
You need Cloud cause lets face it - Ultima Weapon + KOTR + Omnislash = severe pain.
And of course, you NEED Kain, cause if all else fails, you can just have him jump off the screen over... and over... and over... till something dies and you achieve pwnage.
Anyhow... interested to see everyone elses ultimate parties.