According to Reddit, how we should be prioritizing characters for future materia
Tier 1
Cloud - M1 *10%Sword DMG is not too bad either if you have a high star weapon or a Buster Sword. M2 Counter has potential to be very helpful, especially during boss fights. Plus, most likely already one of your highest leveled characters.
Tyro - M1 10% and M2 20% bonus on weakness + spell blade or -ra spells can make short work of bosses.
Tidus - M1 Like most skills that proc when you are at Critical HP, one would hope you never reach this point. But M2 double soul gauge on attack means more likely to have soul break for bosses.
White Mage/Black Mage - M1 is not too bad depending on the chance, but M2 is like honing a spell another level, letting you cast more spells, though usually during trash mob rounds. But if there are more events like Baron Castle that have back to back bosses, that will give you some peace of mind.
Tier 2
Wakka - M1 Chance to preemptive attack depends on how often it procs. M2 I have very rarely missed any attacks, maybe that will change.
Kain - M1 Since only Kain and an unreleased character can use jump, it's not really that useful. M2 Confuse at start of battle is bad, but as some have said just poke the character with a mage staff and you should be ok, then steamroll over enemies.
PLD Cecil - M1 Another critical proc skill, though one that might let you survive another blow. M2 Heal with every attack! Makes autobattling more hands off.
Sephiroth - ATK up to 80% based on party members - depends on if it is the more party members the more powerful or less party members the more power (Assuming it is the latter)
Bard - M1 is character specific, especially since we only have 2 characters that can use instruments. But his M2, while a poor man's bladeblitz, would be helpful for farming low level trash. Not so good when you reach a boss, or when you are trying to sleep one of the mobs in a boss fight.
Tier 3
Rydia - M1 Another Crit Proc, this time Magic based. Almost 75% of the damage in the game is physical. M2 15% dmg for summons is very situational, and if you have not honed summons, it is a one shot deal.
Ranger - M1 Better at bows, which I hardly use! M2 10% chance to do more 280% damage, but I doubt you get to choose the target. With my luck I'd shoot the Ymr Shell every time.
DRK Cecil - M1&M2 do more damage but hurt you. If you have a healer or can salve yourself, it might be ok. Still a gamble.
Tier 4
Tifa - M1 a bonus to her own weapon type! Not great for other characters
Aerith - M1 does bonus holy damage, but have yet to have any really good holy spells. Few enemies have holy vulnerabilities.
Tier 10000
Warrior - No Materia. Keep warming that bench, buddy!
Source:
http://www.reddit.com/r/FFRecordKeeper/comments/3354a9/prioritizing_characters_for_future_materia/