Most versions: Rydia. She is carnage incarnate.
DS version: Rosa. Great physical damage, invaluable magic, and beefed up Pray.
Upcoming version: It looks like Porom is going to be best. Cry halves both physical and magical defense?

Personality-wise for me, it's: Rydia, then Porom, then Rosa.
Appearance-wise, it's: Rydia, then Rosa, then Porom.