stealer
best: tifa (with sneak glove, you have a chance about 80% stealing rate)
worst: barret (he never get to steal one)

Morpher
best: Yuffie (can do 9999 damage with conformer)
Worst: Cait Sith (he heal the enemies!!)

Manipulator
best: Cait sith (With Hypnocrown, I always get to manipulate enemies except boss of course)
worst: Aeris/Red X111 (he/she for me never get to manipulate)

Deathblower
best: Tifa/vincent (God's Hand/Outsider)
Worst: Cait sith (I think he have only 10% chance to do it)

Attacker
best: Cloud/Red xiii (their attack are high)
Worst: Tifa/Aeris (they sucks)

Magician
best: Cloud/Aeris (Aeris suitable for healing while Cloud is good on attacking magic)
worst: Barret/Vincent (they are lot more sutable for long range fighting)

Limit
best: Barret/Cloud/Cid(they can hit for 10-20 times which is higher than others)
worst: Red xiii/yuffie (can only hit for 9999 damage)

that's it. However, this is my opinion. People are different so everyone thinking style is different.
My favourite character: Cloud or Red XIII