Well, as you all know, everyone's final weapon has a special factor that determines their strength... What are they? I know a few:

Cloud: Higher HP
Aeris: Allies near death
Barret: Higher AP materia
Vincent: More enemies killed
Tifa: Limit gauge

But I haven't figured out the rest, I would really like to know if anyone has any ideas as to what strengthen the other final weapons O_o

Red XIII: Higher MP
Cid and I believe Cait Sith also: Same as Cloud
Yuffie: Enemy's level is lower than Yuffie's level (I think this is it, I know it has something to do with the enemies levels. I always seem to get 9999 on hers, so it's a good powerup.)

Hmm... thanks a ton!!!

And I remember Yuffies, which I left out, it's the enemies level, the higher the enemies level, the stronger it gets (so basically, it does LESS damage on weaker enemies O_o)

<font color=red face="comic sans ms">Yeah, Yuffie's basically does more damage on higher level enemies, meaning that enemies at lower levels actually take less damage. But that shouldn't be a problem, considering they have less HP anyway. ;)

And Cid's is based around his max MP, not HP. You've got the rest of them right. :)