Vaan: Bushi/Shikari = HP, tank, damage dealer, mystic armor. (Black Robe: dark elemental damage +50% + Yagyu Darkblade = Yiazmat killer)
Balthier: Time Battlemage/Foebreaker = Breaker, Buffer/Debuff. (Crossbow ranged damage, or close range with hammer or axe)
Fran: Red Battlemage/Archer = Support, Range damage, Access to Ardor (Oil + Burning bow + Fiery Arrows + Ardor = Massive fire damage.. strongest hitting damage in the game)
Penelo: Monk/Blackmage = lots of HP, swiftness, access to Holy magick, access to Staff of the Magi: +50% Holy Damage (Blackmage with Holy? Holyshit!)
Basch: Knight/Bushi = Tank, lots of HP. access to white magick, mystic armor (White Robe: holy elemental damage +50% + Excalibur = God combo Undead killing machine)
Ashe: Whitemage/Machinist = Machinist guns adds a fixed amount of damage, access to time magick and lots of HP for survival.
there is no right or wrong, I choose this strictly for making trial mode alittle bit easier.