I haven’t played any earlier ones as they’re a bit old for my tastes. I’ve completed Ys VIII and played some of Ys IX but couldn’t say much about it. When I searched the same question before I started, the consensus seemed to be start with Ys VIII then Ys IX. The games aren’t in chronological order though so just play what seems interesting.

The newer one, Ys X Nordics, I also haven’t played due to the controversy surrounding it (they released it in Oct 2024 then a couple of months later announced a different version that was released in Feb 2026 with more content and QOL added - if you wanted the update you had to buy the whole game again!).