I really can't help you with pre-ending stuff, seeing as I've been post-credits now FOREVER. I do remember that I used Lightning, Hope, and Fang before the story ended though.
Lightning: wins epically because she's an awesome Ravenger and an okay Medic; She and Hope can team up to get the party back while Fang takes the blows as a better than average Sentinel. She and Hope can team up again as Ravengers with Fang as the Commando during the many times you'll use Relentless Assault.
Hope: is the best Medic in the game. Period. I guess you could switch him for Vanille at the start, but then you miss out the better than avergae Synergist qualitites he brings to your team (not as good as Sazh, but Sazh sadly sucks at everything else). I already mentioned his Ravenger usefulness.
Fang: is the best Commando in the game. Period. She's also the second best Sentinel behind Snow, and Sentinels are VITAL if you want to keep from getting thrown around in the late chapters. Great for Mystic Tower and Diversity.
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For post-game, I find myself using the "specialist" that I kept all the CP to through the entire game. I never touched Snow, Sazh, or Vanille after the character swap option was available, and when the game was over, channeled all their CP into ONE STAT. Yep, ONE STAT.
Snow: Being my favorite character, I love how usefully he is post-game. An excellent commando right behind Fang, you've got a good fighter that'll hold his end of the tab during Relentless Assault. But since he's THE BEST Sentinel in the game (and that's the job he;s now fully maxed at), he's absolutely vital to take all the damage. A good fighter and damage sponge = WIN WIN WIN.
Sazh: Poor guy; he pretty much sucked the whole game except for getting Haste so early. But that's all turned around, now that he's maxed out and is the best Synergist in the game. Possessing every state boost you can get, he can't be beat by anyone in the department as far as diversity to the stats go, be it from Protect to Haste (btw, Haste makes everthing sooooo much easier). He still sucks offense wise, but he'll hold up his end as the okay Ravenger during Relentless Assault.
Vanille: I actually maxed her out in Medic, despite the fact that her strongest suit is Saboteur. You see, I won't have Hope maxed out in Medic for a very long time, since he was at 0 CP after the credits rolled. But even then, Vanille is now preferred BECAUSE she's the best Saboteur. Being the second best medic is just a nice little add-on. Having Sazh boost stats while she lowers your enemies' is the best way to start a battle most times. I love Bully the most really.
EDIT: And wow, that post was way too long......
