Ruby

Start with 9 as it's clearly the best in the series, skip 1-3 as they're not very good, 3 could be good if they update the job system to be like 5's but they haven't so it's not, play 4 to see the first proper attempt at storytelling in the series, play 5 for its excellent job system (currently playing the GBA version now, gotta say massively underrated story and world).

Skip 6 because its an overrated mess of a game that feels like a dozen bad fan fictions jammed together, the world is such a disjointed mess, a random opera house nowhere near any towns, an arena nowhere near any towns, the only railway line seems to exist to ferry people to the afterlife but people don't question using it as a mode of transport even though they can't have seen one before, there's a castle that can dig through sand which somehow is also able to go under mountains, the closest town to that castle isn't part of that kingdom, neither is the town the castle can dig through bedrock to appear next to, instead the only town that's part of that kingdom is separated by a large mountain range and connected by a narrow, winding cave system.

7 is overrated, narratively a mess but still pretty good, the first few hours are excellent. 8 can be skipped, I do like the opening but there isn't a single likable character, some of the mechanics are good but they're also broken.

10's good, pretty much the only time that summons are worth using, decent story, enjoyed it. Skip 10-2 unless you're a j pop fan.

12 is a good RPG but doesn't qualify as a FF game in my eyes. The characters and story might be the most bland since FF2, the world and lore are pretty good, combat would be fine but the gambit system is massively undercooked, giving you the tools to code your own AI but only giving you 1 "if" line per prompt.