OK first off all I have a question, what stat affects steal rate? Is it Dexterity or Spirit? Anyway:

Stealer
best: Yuffie (has high dex)
worst: anyone else with low dex

Morpher
best: Yuffie (can do 9999 damage with conformer)
Worst: Aeris (her attacks are weakened by overabundance of magic)

Manipulator
best: Cait Sith (hey its his materia)
worst: Never tried it with anyone else

Deathblower
best: Tifa & Vincent
worst: Cait Sith

Attacker
best: Cloud & Cid
worst: Aeris (see next option)

Magician
best: Aeris (She can be a summoner too), Yuffie (Aeris: again overabundance of magic materia; Yuffie: when Aeris leaves the party)
worst: Tifa

Limit
best: Cloud, Cid, Aeris (The two hits for massive damage; Aeris has some pretty useful Limit Breaks)
worst: Cait Sith, Tifa & Vincent (I hate slots, they're too random; I hate hers, they can miss; I hate his, I cannot control him)

My favourite character: Aeris