Favourite - Rydia, very similar to Terra, at least in appearance, had a great story, in battle yes in the end parts of the game, the most damaging combined with Bahamut and Dualcast just amazing quite frankly but, she isn't a white mage, Rydia is the 2nd most useful character IMO.

Most Useful - Rosa, When you are without a proper White Mage in the game it becomes obvious pretty quickly that Rosa's role is really important, combined with Fast Talker, Omni-casting and Dual-casting she is a boss, Haste and Blink is always a good combination once you have the correct augments, yes can't really do damage until late in the game but even holy does some kick-ass damage for a white mage, her story was good-ish she was a stereotype princess role IMO, Rydia had a better story than her but I'm not complaining.

Porom - Wahh Wahh!