The Ys series is that series for me that simply made me think "why the hell didn't i play these games sooner!?"
Ys Seven was an awesome game, and my first Ys game. Charming characters/story, fast paced and frenetic gameplay, and AWESOME soundtrack had me hooked. After beating Seven, I found out 1,2, and 3 were also being released on the PSP, and I scooped those up in a heartbeat.
Once again, I was not disappointed. They were extremely fun and challenging, and seeing the evolution of the series up to 7 was great. I researched the franchise, bought and enjoyed Origins and even re-purchased 3 off of steam as well. Hunted down a copy of Ark of Napishtim for the PC with translation patch.
This series has been around for a wile, and I never really played them until 7. This series was one of the founding fathers of the action RPG, and I'm glad I got a chance to play them, save for 4 and 5. Have all the sound tracks too, because Falcom JDK are FUCKING AWESOME.
I COULD play 4 right now, but I'm eagerly awaiting Ys: Celceta (the true remake of 4) localization. One of my most anticipated Vita games.