LogFAQs > #980076899

Gameplay order/release order. Genuinely.

With Steam's offerings, you go in this order:
I+II Chronicles+ > VI: Ark of Napishtim > Oath in Felghana > Origin > Seven > Memories of Celceta > VIII: Lacrimosa of DANA > IX: Monstrum Nox > X: Nordics (coming later this year)

Good luck actually beating I, though, the final boss is genuinely bullshit and it might be better to Youtube the ending if you can't get beat it.
VI is very rough, as a warning, being the first one of that gameplay style with some certain very obnoxious aspects.

Don't worry about the games referencing to each other. In this order you get pretty much all the context except for VIII and IX's references to V, which is only on SNES and desperately needs a remake, and notoriously considered the worst Ys game anyway so no harm in skipping it.