Pantheon = Kain Highwind (or any Dragoon)

Kefka Palazzo. Only villain in the series who successfully destroyed the world.

Auron. He pretty much said '**** you death, i'm hanging around a bit longer'.

Yuna
If you die once, she'll send you.
No more respawn.

Anyone from FF10 with even decent progression on the sphere grid? 99,999 damage per attack, attack 10 times before you can attack once.

/forumrate foodflavour

Coolest would be Gau.

Let's see, a few are already in.
Tifa = Vi
Cloud = Riven
ViVi = Veigar
you could argue that any 'summoner' = Annie. Big honkin nuke/summon ally.
Auron = Tryn?
Barret = Graves (in playstyle anyway, tanky adc)

I think Terra would be pretty nuts. She'd have some close range capability, enough AP nuking power, and her ult would prolly put Nasus and Renek to shame. Esper mode would blow up everything.