Cecil(Dark Knight)- I have to say, Darkness really helped for bosses.
Cecil(Paladin)- Without the Ultimate weapons, I would have never beat Zeromus at a lower level than I was supposed too.
Kain- Jump saved my life hundreds of times.
Rosa- She would have been better with some blk magic too. Pray helped too.
Rydia- Her summon, Bahamut, saved me during many boss battles.
Edge- Throw rocks! I had 99 Fuma Shurakins and I defeated Zeromus by just throwing them!
Yang- Kick just was cool. I was able to defeat enemy parties that I was outnumbered in in just seconds.
Edward- LOLed with the hide. Salve was very useful.
Palom and Porom- Twincast! Twincast! Twincast! The best move they have!
Fusoya- Hmm, everytime I leveled up, his stats didn't level up as well. So I consider him useless but helpful for the Archfiends battle!
Tellah- His Blk and Wht Magic helped a lot. Without it, never would have defeated the Magus Sisters.
In the end...
Top 3:
Edge
Kain
Edward