Yuna,

Because she will summon the Magus Sisters, who want revenge on Rydia and her comrades for tromping on them back in FF4j.

And let's disregard the Sphere Grid, m'mkay :rolleyes2 .

DUN DUN DUN

Cloud vs. Squall.

Oh, I went there. And, while I may be giving the most nonsense reasons I can think of for my answers, let's not all pick Cloud over Squall because "FF7 is r0cks0rS! an ff8 si teh L4m3!11". I'd pick Cloud over Squall, too, but I like FF8 better.