I'd love to play this on an iPad to see the full detail of things, but I only have a small iPod touch for the moment. Having FFT on iOS would give me a good incentive to consider stepping up to the iPad.

My pick:

Setting aside practical use in battle, I've always had a guilty pleasure in using Calculator/Arithmetician. The game isn't ever very good about giving you easy situations to use it to your advantage, but when the maths work out, it's satisfying. I really like the idea of incorporating sciences into a fantasy setting (yep, I like steampunk), and Arithmetician helps me bring out my inner math geek. I'm also a big fan of the character design for this class.