Well, going by FF IV, VI, IX, and XII (and a bit of X)...

Coolest outfit: Blank.
His giant headband is pretty sweet, and the rest kinda complements it.

Most practical outfit: Cecil, as a Dark Knight.
Big suit of armor. That's all there is to it.

Most boring outfit: See above.

Most impractical outfit: Fran.
Running around in the jungle with high heels? Yeah, that's smart.

Best formal outfit: Celes' opera dress.

Best uniform: Palace guards' suit from Chrono Trigger.