Well you have to go with Rosa.

She's the female lead, she's a pure white Mage, and she's Cecil's girl.

Rydia is awesome. A great story. I love how you find her, lose her, and she comes back much better than before, but she's a crossover mage. An overall great tool for the party, but not the meat and potatoes of it.

Porom and Polum are super cool, but much much much more tertiary characters, and neither is realy complete by themselves. They did have one of the very coolest scenes in all of FF. I didn't know if they were ever come back, and I ventured into that castle a number of times to see them. It's almost like I felt the need to pay respect to them and I was truly waiting to find out what had become of them, but still, they don't place above Rosa.