Cyan- Best character in the game (without relics) if you raise him right (with + strength espers). Once you get the spell Quick the charge times for his sword techs no longer matter. Plus, in my game I had his quadraslam, not quadraslice mind you, dealing about 9k damage a pop. Times that by 4, and I think you can see why I like him.

Sabin- Early access to bum rush works wonders. After a while any blitz is worth a solid 9999, and so he is a good guy to take. Plus you always have mantra and spiraler if you get into trouble.

Locke- Here is where the relics come in. Load this bad boy up with Illumina, Atma Weapon, Genji Glove, and Offering. If raised right, you will unleash 8 attacks that do just about 9999 each. Not to mentiond having like a 98% in both evades. 'Course I still like Cyan better...

Edgar- This one is variable actually. All you need is someone with maxed out magic (using espers again), a gembox, and an economizer. Who you use doesn't matter since whoever it is will bring forth 9999 with ultima every casting. I take Edgar because I happen to like the Figaro bros.