FFI- I'd go w/ a party of Fighter, Monk, Monk, White Mage & Black Mage. Having the extra Monk would be some nice extra firepower to the basic party.

Skipping III because I've only completely played through the game once.

FFIV- I'd add FuSoYa because his Sage abilities would be excellent. If I could replace Edge & add two, I'd go w/ the Twins instead.

FFV- I'd have Krile send Galuf to T.A.H.I.T.I., so he could rejoin the team.

FFVI- Locke, Terra, Celes, Setzer & Sabin.

FFVII- Cloud, Barret, Tifa & Vincent.

Skip the next two because I can't think of them right now.

FFX- Wakka (for Avians), Auron (for the thick-skinned ones), LuLu (for the ones weak against magic) and Tidus (for quickness). This would leave Yuna in reserves for surprise Malboro attacks, so she could summon meat shields.

FFXII- Penelo, Ashe, Vaan & Basch. Penelo for Magic/Range attacks, Ashe for support & fighting, Vaan for attack & Basch as the Decoy.