For me, I somehow ended up using all the Black Magic spheres I recieved to Yuna, so once I started using Black Mage skills for her, along with her White Mage and summoning skills, she ended up becoming my strongest character. (Well, until she's silenced that is...)

The order of strength and Magical strength for me would have to be:

-Yuna
-Auron
-Tidus
-Kimahri
-Lulu
-Rikku
-Wakka

I never could get any better at Blitzball, so Wakka just kept getting worse and worse.