An Ys-y subject.

I've beaten at least one version of each game in the mainline series - I haven't played the 8bit versions of Ys (though I do own the first game on Master System), but have beaten Ys I & II twice in the Chronicles version, and Ys I once on TGCD (I'll do II eventually). Never beat or even played much of the original version of Ys III, but absolutely adore Oath in Felghana (which is the game that got me into the series on PSP). The only version of IV I've beaten is Memories of Celceta, I've beaten Ys V, VI (PC), VII, and VIII (once on Switch, once on PC). Haven't played IX yet, but I'm sure I'll like it.

My favorite is Ys Origin, which I've beaten with each character twice, and probably will attempt the harder modes eventually. I just think it's incredibly well put together both aesthetically and in terms of gameplay. The third character you get might be my favorite character to play as in the entire series.

Ys V is my least favorite, though I haven't played the strategy game or many of the three billion ports of the first two games (I suspect the DS versions of I & II could be my least favorite, since I've heard they play very badly). Ys V is a good game, but there are others I'd rather play many times before going back to it. It was a bit slow paced and super, super easy, so much so that they apparently released a harder version later that hasn't been fantranslated as far as I know.

Play Ys! Play Falcom games in general! Trails in the Sky/Zero/Azure/Cold Steel are all great (if insanely long)!

I'll be honest, I didn't love the first three games. Of the games I've beaten:

Ys I (DUO) - Was getting into a groove, but so much of the game is taken up by the tower and there's a horrendous backtrack needed there that brings it down in my mind. Eh/10.
Ys II (DUO) - Shifts the combat away from bumping to magic projectiles, and feels a bit more balanced, but I still don't love it.
Ys III (DUO) - I want to love it, but it's still quite rough around the edges. And geeeeeeez the voice acting is awful.
Ys V (SNES) - Quite easy, but solid. Would like to play through Expert.
Ys VI (PS2) - Janky as all get-outs, but this is the game that got me into Ys. Fast, frenetic combat.
Ys: Oath in Felghana (PC) - And this is the other one. Refines the formula in Ys VI, and proves one of the best remakes of a game out there.
Ys Origin (PC) - More of the same. Thought I wasn't going to like everything being in the tower, but it works. Three player characters can be somewhat repetitive, but the game is good enough to justify playing through all of them.
Ys Seven (PSP) - My former favorite of the series. Miss the jump button, but it does a great job of updating the formula again.
Ys Celceta (Vita) - Fun, but not as good as Seven for me.
Ys VIII (PS4) - Probably my favorite of the series now. Crazy fast combat and exploration elements that open up as you progress. I could do without the defense segments, but otherwise one of the greatest modern ARPGs.
Ys IX (PS4) - Small downgrade from Ys VIII, but still very fun.

So basically I've finished every game in the series in some form. Still wouldn't mind running through the Duo/SNES versions of Ys IV, though. I have played both of them.

The music owns hard.

Reminds me, years after I got the game I figured out that Eternal had the soundtrack's sheet music on a bonus disc. Yeah I could have just found this on the internet but it was still cool.

I love the so-called bump system. Being able to just run through enemies and level up in seconds was so liberating at a time where I was always playing RPGs. I found the first two games a bit overrated - they're very 80s, not quite developed enough for me. I liked Ys III on the Genesis, enjoyed both versions of IV a lot, V, VI, Origins. For me what was so great about the early games is how concise and fast they were.