Since I'm a lazy bastard I decided to copy one of my older posts, I think it fulfills most, if not all of your requirements. Though neither one of these characters seem fitting to be a leader, since I have to chose one I'll go with Basch who has immense experience in that field.
Squall - Beside the fact that his limit breaks deal massive damage the junction system allows me to mes with his stats however I like, which means I can approach every battle in different way. Not to mention the neat abilities he can equip, be it for raising his stats even more or just for supportive purposes.
Terra - For the most part the player itself watches over Terra's growth, which means you can create a character with some neat stats. She can pack quite a punch (physical attack) which isn't really that common for a girl in FF franchise, also her trance ability proved to be more than useful.
Quistis - Same as Squall, at least when it comes to junction system. Though I'm one of the rare ones who think she has better limit break. She's the one who saved me whenever I was in pinch, her Blue magic is quite useful, especially for support purposes.
Zidane - You gotta have a thief in your party, there's always some neat treasures just waiting to be stolen and who's better in doing that than Zidane. Also, he's probably the only thief who can deal some serious damage, without messing with his stats of course, though I must admit I'm not really impressed with his Trance ability.
Lulu - The best black mage I've seen so far, not that it has to be her only class (great white mage for instance), but we'll stick to it. Doublecast ability combined with her celestial weapon does some impressive damage, also we need to keep in mind that every spell costs one mp even.
Yuna - Her summons are simply amazing, Magus Sisters killing Omega Weapon in one hit isn't something you see often. As for the rest, same as Lulu.
This is based solely on gameplay, personality wise it would look somewhat different.
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