My favorite party is:

Warrior
Monk
Red Mage
White Mage

I find having two magic-only mages becomes a liability due to their low defense. I chose WM over BM as White mages are able to use the Heal spells, whereas the RM cannot, and Heal is absolutely necessary when fighting Chaos, IMO. Monk, despite his low defense, does a lot more damage than the Thief (actually there is not much difference in their defense to me). He is also a lot cheaper to equip.