Amarant was really useful because of Chakra. Vivi however can really enhance your party, especially with Steiner. I wasn't really into the White Mages last time I played, basically because they're like the same character, just with different summons. It also seems that 9 is the game that made it a rule that the White Mage also has to be a summoner. I dont like that, although it was helpful advice for playing FFV.
Freya's good too cause she's definately the Red Mage of the party. I think next time I play I'm gonna really try to get Quina real strong.