I never put the proper time to build up Tifa to be as strong as she could be, so I missed out on her potential. On my next playthrough, I want her to be a big focus of mine. Worse yet, I had no idea Barret had melee options! Damn, that's crazy.
Cloud is my favorite currently, just because he's so reliable. Punisher mode is very satisfying to pull out strategically. It can be very ineffective if you use it at the wrong time; if you use it at the right time, it's crippling to the enemy.
Aeris, as a ranged character, has a trait I really like - in risky situations, she can build up ATB without having to get into the fray (Barret also has this trait). Because Aeris has a couple of really good moves that cost both ATB bars, I enjoyed playing her. Especially that one laser move, the name escapes me right now.