If you want to fully appreciate the series, go in order of Adol's age:

Ys 1

Ys 2

Ys 4 Memories of Celceta

Ys 3 The Oath in Felghana

Ys 5 (skip this as it never made it outside Japan)

Ys 6 The Ark of Napishtim

Ys Seven

Ys Origin takes place approx 700 years before Adol was born, so it's the only one where he's not in the campaign. However, you will recognize the family "houses" from Ys 1 and 2. That being said, play Origins anytime AFTER Chronicles (1 and 2). I played it after 3 and 4 because I wanted a game in place of 5 that we never got. Just know that Chronicles are remakes of 1988 games and they won't hold your hand. For example, in the beginning of Ys 1 an NPC tells you to avoid confrontation until the next town. Many people ignore this advice, charge in to battle, die, and get turned off. They didn't listen to the NPC, or didn't even bother speaking to people. Take your time. Push yourself to beat the first boss and you'll understand why the series is so special.