I stopped with the fourth one. It was a good length adventure with lots of content, balanced grinding, nice music, and a satisfying ending.
I did try the remakes years later, as well as Ys Origin, but just couldn't get into them. I also tried Ys V in recent years, but after the Turbo version of IV, it just felt like crap. I do have a later entry or two on my list, but no rush to get there.
I finished III after all these years, and I'm currently at Darm Tower again in the original, and will play IV again after that.