You're going to get every possible answer here.

Watch some videos on youtube and just pick what you think looks fun. There's no wrong answers here as all the games recommended in this thread are great games.

I've played 7-10 and 12. And my order goes 12>9>7>8=10.

The thing I liked about 12 is that the plot is much more mature and realistic than those of other FF games, and I loved how strategic the combat became as the game progressed. The game uses a system where you "program" when your teammates will do what, and I had fun tinkering around and adjusting my battle plan as I progressed through different areas. The game also has a much larger world and far more content than any other FF game on the switch, so you'll get a lot of bang for your buck