Like this:
1-5. Original era. Classic style and presentation, high fantasy setting. Classic good vs. evil plot.
6-10. Big Budget era. Presentation evolution to 3D, more tech-based futuristic setting. More convoluted plots with grey areas and hidden moral messages.
11-15. Post Sakaguchi and Uematsu era. Loss of classic style of JRPG in story telling, gameplay and music. Dying of franchise with loss of original inspiration.
Within the titles, I would separate them into style of play. Typically there is an a-team and a b-team with different styles, alternating. One team is more linear and story-driven, the other is more open and tends to use the "jobs" system more. FF1, 2, 4, 6, 7, 10 tend to be a-team titles, while 3, 5, 10-2, 12 make me think b-team. I'm sure I'm a little off in terms of which teams made which, but I know it went something like that.
