Tank - Vaan or Ashe. Sword and Shield, and, if I remember rightly, I would usually set them up with buff magic. Protectga, Shellga, Hastega, Lady Gaga etc.

Damage Dealer - Basch or Balthier. I got the Kotetsu from grinding Werewolves very early in the game, and Basch just annihilated everything with it. He's a beast with big swords. I also gave him Arcane Magic, so he could cast some pretty brutal spells on enemies to complement the physical pounding. (Once again, not completely certain. I last played this game years ago, so I can't remember everything haha)

Support/Healer - Penelo or Fran. The White Mages of the group.

I would then balance out the other magic schools where I thought they were thematically appropriate.