Predictably, since my root Final Fantasy is X, I lurve me some black mages.
No, not the cleavage. The firepower. All my battles look like this by the time Rikku joins the party:

Tidus: *ping* 203HP
Kimahri *steal*
Wakka *thwock* 400HP [one monster is now ded]
Rikku: *steal*
Yuna: *pray*
Auron: [OVERKILL 1200HP]
Lulu: [THUNDARA 2200HP]

Neeeext....

Vivi delighted me as well.
FFXII was weird. I kept trying to have a dedicated black mage, since that's my default method of dealing with problems, but I wound up distributing all the magic somewhat mushily, and didn't use any of it very consistently.