Best games:
-Ys: Oath in Felghana (PC) - probably most peoples' pics for favorite Ys game
-Ys Origin (PC) - one of my absolute favorites in the series, a precursor to Adol's adventures (it doesn't even star him) all confined to Darm Tower, but it's the ultimate distellation of all that makes Ys good
-Ys IV: The Dawn of Ys (Turbo Duo) - the best of the oldskool run-into-the-enemies games, spectacular soundtrack too, probably my all time favorite Turbo game
Good games:
-Ys 6 (PC/PS2) - the first game to use the engine seen in Origin and Oath in Felghana. Not as polished as those games but still really good
-Ys 7 (PSP) - takes away the jumping and replaces with a roll, game seems a bit more drawn out compared to other Ys games, and it has lots of talky dialogue
-Ys Oath in Felghana (PSP) - not as good as the PC port since the visuals are much more difficult to make out on the small screen
-Ys I and II (Turbo Duo)/Ys I and II Eternal (PC) - you can't go wrong with either version, the PC one isn't as fast and responsive as the Turbo, but has better graphics. I do prefer the Turbo soundtrack since it's a bit more 80's synth and less orchestral than the arrangements on the PC version.
-Ys Memories of Celceta (Vita) - has some issue with slowdown and pacing due to its large, maze-like areas and wordy story. Not to mention it expands on the crafting and loot gathering that got its start in Ys VII. Not a bad game but it lacks the brevity that makes the Ys series so attractive
Meh:
-Ys III (Turbo Duo/SNES/PS2) - incredible soundtrack, gameplay has been replaced by simple side scrolling
-Ys IV: Mask of the Sun (SNES) - a completely different game from Dawn of Ys and Memories of Celceta, and the weakest out of all of them. Forgettable soundtrack, gameplay doesn't feel as refined as the others.
-Ys I and II (SMS/NES/DOS) - certainly playable, but if you want oldskool Ys you're much better off with the Turbo version
-Ys V (SNES) - brought jumping and sword-swinging into the overhead Ys template, but visuals/story/music feel more like a forgettable SNES action RPG than a Ys game. Pretty short, sluggish movement, and lacking in the feel of adventure or the great boss fights of Ys.
-Ys VI (PSP) - extremely slow loading times, maybe if you pirate it it won't be so bad
Haven't Played:
-Ys I and II (X68000) - what's interesting is the art style in this version, it looks like a western RPG and has digitized actor portraits in lieu of anime art
-Ys I and II (Saturn) - seems more of the same but replaces the vivid colors of the other versions with a more reserved, grittier color palette
-Ys III (Genesis) - not sure how this differs from the Turbo or SNES versions, but I'd assume it has the worst music of the 3 at the very least. The rarest of all the Ys III games, too.
-Ys VIII (Vita/PS4) - initial Japanese reviews seem to be fantastic, but it's also the longest and most story-heavy game in the series by far (that Kiseki influence...)