Ys I & II Chronicles are pretty different from Oath in Felghana and Origin, so it'd depend a bit on your taste. If you want something extremely retro, I & II are really fun games and you'll definitely relive some of those "16-bit" moments with them. On the other hand, I don't think they're a very good representation of what Ys's turned into nowadays. For that, you'd definitely want to go with either Felghana or Origin. And if picking one of those, Felghana's probably the best one to start with, because Origin is a prequel to I & II and it also doesn't feature Adol, the series' main character from like, every other game.
Soo... I guess I'd vote Felghana. If you like that, then on the next sale you can pick up both I & II and Origin, and play them in order, haha. They're all mostly standalone though, even Origin and I & II can be played without having played the others, it's just you'd be missing on a bunch of references and what not. But the gameplay's so solid and the main point of it anyway, so...