Sabin and Edgar for two reasons:
  • Sabin and Edgar find unique and powerful (for this point in the game) equipment (Fire Knuckles and Chainsaw) while Cyan, Gau and Shadow don't.
  • Edgar's Auto-Crossbow is still powerful and the lack of attacks or magic that can hit all enemies makes the Auto-Crowssbow much more conventient. Same goes for Sabin's Fire Dance, which I usually learn before or while in Zozo.

I still mix it up every now and then but Sabin and Edgar are my main choices.