I actually just answered this on the Steam forums not too long ago, so I'm going to copy/paste my response here. Apologies for the "lazy way out," but hopefully you'll find this informative -- and if you have any further questions, you may certainly feel free to ask, and I'll do my best to address them.
The Ys series is a little different from most, in that it's completely episodic -- you don't have to play the games in order, since each tells a self-contained story. Besides which, story isn't really the draw for most Ys titles anyway; it's all about the gameplay.
That having been said, here's the quick rundown:
Ys I & II Chronicles+
- Remake of the very first two games in the series, originally published in Japan in 1987 and 1988. Old-school to the extreme, with a lot of obtuse puzzles, a classically-styled fantasy storyline, and gameplay that's decidedly not modern at all (but is very fun if you give it a chance!). A good starting point for retro gamers, but might be better suited to those who are already fans of the series, and is definitely not recommended if you've been weaned on modern video games.
Ys: The Oath in Felghana
- A complete reimagining of Ys III, though its connections to the other Ys games are tenuous at best (read: it's one of the most standalone games in the series, story-wise, spoiling nothing and requiring no prior knowledge of the series whatsoever). Considered by many to be the best Ys game, myself included, though its high level of difficulty may make it a little less accessible to gamers who aren't used to challenging games (fortunately, there are 6 difficulty levels to choose from, so you have options to make life easier for yourself!). This would be my recommended starting point, however, as I feel it best represents the series as a whole.
Ys Origin
- The earliest point in the series story-wise, taking place 700 years before Ys I & II. However, it's also the only Ys game that doesn't star (or even really feature) series protagonist Adol Christin (since, you know, he's not born yet!), and it's also the only Ys game to feature no overworld and no towns -- the whole game takes place inside a giant tower. Feels very arcade-like, but also happens to have the most complex and interesting storyline in the series, as well as some of its best gameplay (though its map designs aren't quite as good as Felghana's IMHO). May be a good starting point if you prefer your games more straightforward or really want an engaging story to focus on.
Another way to look at it: Chronicles+ feels kind of like an old-school Zelda game in fast-forward, Felghana feels kind of like a 3D Metroidvania done right, and Origin feels kind of like an old-school pre-SotN Castlevania game in 3D (again, done right).
Would you say the Ys games are more story heavy than mechanics heavy?
Nope! Totally the opposite, in fact. Except in the case of Origin, where there's a much greater balance between the two.
-Tom
