I've recently grown very fond of the party Fighter/Fighter/Fighter/Red Mage.
All characters can deal good damage with physical weapons, plus the fourth one is a Red Mage and therefore knows a repertoire of white and black magic.
I think this is one of the best parties, and definitely one of the most enjoyable ones.

The worst party is probably four Thiefs, because while the White Mages are weaker, at least they can heal themselves and use EXIT.