I choose my favourite characters, think about convenience and what I think is stylish. The two protagonists if there are two are always included. Also, because I think they need to be included as the main characters and because I normally also like them the most.

VI: Terra, Setzer, Celes, Gogo (before you get Gogo Locke, Shadow or Cyan)
VII: Cloud, Vincent, Aerith/Tifa/Yuffie
VIII: Squall, Rinoa, Irvine
IX: Zidane, Sarah, Steiner, Vivi/Eiko
X: Tidus, Yuna, Auron/Rikku
XII: Vaan, Ashe, Fran
XIII: Lightning, Fang, Vanille/Snow/Hope