@eisai Play the first two games. The bump-combat system isn't bad. I recommend either the TG-CD version or the PSP remake (also on Steam and GOG).
Skip the original Ys III. It's a shitty Zelda II clone. Play Oath in Felgana instead. It's considered to be the best in the series.
Ys IV has three different versions with completely different stories: Dawn of Ys for the PC-Engine CD, Mask of the Sun for the SNES, and Memories of Celceta for Vita and PC. Any one of them (or all of them is fine).
Ys V is considered to be the weakest in the series. It was only released in Japan on the SFC, but there is an English patch. The game isn't bad, but it is much slower pace and the music is subpar due to it being on a cartridge based system.
@eisai It feels like nobody gives enough attention to any of the games before VIII mostly because Falcom has always been obscure outside of Japan. I honestly prefer the older (pre-Seven) games due to their simplicity, but unless you grew up with a TurboGrafx or rented Ys III on the SNES or Genesis, you probably never even heard of the series until VIII.