IX is the main title Sony-platform Final Fantasy game for me. It takes some of the things they learned from FF7 and 8, and applies it to an oldschool FF setting. It’s closer to FF 1-6. I’d say it’s closest to 1, 4 and 6.
I am Setsuna is a short little RPG similar to the older Final Fantasy games. I recommend it if you’re looking for some old school Square stuff. It doesn’t have the depth of Bravely Default, but it’s a good way to get back into the groove.
