Play Order:

I Found this site and unless someone tells me Otherwise it seems like a good Order http://www.digitalemelas.com/index_ys.php

Ys 1
Ys 2
Ys Origin
Ys 4: Celceta
Ys 3 Felghana
Ys 6 Napishtim
Ys Seven
Ys 8 Lacrimosa of Dana
Ys 9 Monstrum Nox