Mine has to be Choco/Mog from VII.
Their failed attempt at a kamikaze attack (The failure is that they both inevitably survive it) provides a welcome touch of comedy in an otherwise intense boss fight, and their considerable chance of temporarily stunning the enemy means that they remain useful long past the point where their base damage seizes to make much difference.
There is also the fact that, once in a blue moon - usually about twice during an average playthrough - it won't be Choco and Mog who show up to save the day. It will be Fat Chocobo, descending from the heavens ass first and leaving all the work to gravity.
I'll never forget the time that happened while I was fighting Yuffie. "I'm an awesome ninja! Your materia belongs to ME! FEAR ME!" And then the sky darkens, and WHAM! Next thing I know, the entire screen is taken up by a morbidly obese Chocobo with an expression of solemn dignity and an assful of wannabe ninja bimbo. That was probably the biggest laugh I ever got from that game.