Quina is the best character after Steiner. I cleaned up Ozma with a party of Zidane, Steiner, Quina and Amarant several times. No need for a white mage with Quina and Amarant. Quina can perform Frog Drop for max damage and has access to support magics like White Wind and Auto-Life (!). Quina is by far and away the most flexible party member.And FYI Blue magic in the odd number games is over-powered.
You're right about Kimahri and Selphie though - blue magic in 10? Yikes. Plus, Kimahri can't swim like Wakka, Tidus, Rikku or summon like Yuna and each of those four had far better overdrives. Farming Dark Yojimbo is so much easier with Rikku's slots. And Selphie's slots just isn't up to scratch with Squall, Zell, Irvine or Rinoa's limit breaks. The End doesn't appear frequently enough even at max crisis level to be useful.
5,6,7,8,10,12 would be contenders but are hard to choose from. 5 has such flexible gameplay, 6 has all kinds of different characters, 7 and 8 have complex machanics you can set up differently every time, 10 is epic, and 12: Zodiac Age is a bit of all the aformentioned. All the others are either too linear, too simplistic, or too primitive in design.
