For me, well now that I'm in post-game, I use the three girls, where I have Fang or Lightning as my party leader and Vanille as either medic, sabatour or synergist, but I'm also working on other diagrams aside from the three they started with, those are maxed.

What I've come to find out is when Lightning has Sentinel along with the skill Elude, and the accessory Nimbletoe boots, she becomes pretty untouchable. If you finish her diagram and get to Reprieve, it allows you to survive with 1 hp if the attack would have otherwise killed you. So if she gets hit she will still be alive.

Fang is best suited for Commando due to her Highwind ability which works best with a high str stat and a staggered enemy.

Vanille is pretty good when it comes to medic, her magic stat is high and for some reason I prefer her over Hope, maybe because she looks better