If I could have a class in FFT, I would be an Oracle. In my opinion, Oracles just rock! As previously said, Oracles are way underrated by too many people. I can also do many things Oracles can do, just w/o magic. You know, the good stuff, like making people go mad, raise peoples' faith in God, and make them reluctant or afraid to do something. As a side skill, I would have a mastered Math skill with all the important magics learned on the other classes like: Flare, Cure3, Raise2, Shell, *Edit* Demi2, and Haste. I'm great at math -top of my class- and find it much less boring than any other subjects in school.
On a side note, all of my main party ('cept my physical based guy) have a mastered Math as their side skill. We kick everyone's arse every single battle - sometimes after just one round of turns!
My other abilities would include: Counter Magic, Magic Attack Up, and Teleport. Yup, I'm a mage-based guy.