I've played every Ys game in some form and also finished most of them. VI I couldn't finish because the dash jump maneuver was so annoying and mandatory to play the game, thankfully Felghana has a regular double jump. V is infamously a piss easy game, so it wouldn't have been too much effort to finish it, I just found the art style terrible and the combat too boring to continue. I got annoyed at some boss fight in the PC Engine IV: Dawn of Ys and stopped playing, but Celceta is now the canonical IV anyway. Still working on X, but I'm loving it so far.

VIII is by far my favorite in the series, probably followed by Celceta or X, Origin and I&II. I think most entries are at least worth playing.