(Sorry if this is too much text :/, I won't be offended if you don't read it.)
Panaias: As I said in my first post in this thread, I managed to complete I & II using a walkthrough 3 or 4 times in total. Oh, and I still remember those mazes... At some point I used a piece of paper to draw a graph while moving, otherwise I would never be able to progress in those caves!
That is rather good, I used it more than that and only partly because I was too lazy to draw :(. Speaking of caves, did you run around the warm cave area as a roo and talk with the monsters? The one who wanted to join the monster union makes me laugh. The temple complex is the part I had the most trouble with.
I think all of the older ones I ended up using a walkthrough a number of times, although the newer ones I think I only checked once or twice if at all.
Panaias: Due to the hardcore nature of Ark, Oath and Origin, Mr.Mumbles previously suggested that I skip those and try from Ys Seven onwards. You seem to have another opinion (I like when that happens in a healthy discussion) :)
One aspect is that Seven is not as polished as Memories and is in that way harder to play later. I only remember the tie in with VI at the beginning and that is minimal so I think it is slightly less connected to the others (and the connection is never more than slight in any case so not playing story order is completely fine). Also, if you play VI far enough then there is a tie in with V that is more extensive than most if not all of the others (still minimal) and V only has a fan translation so far. Memories takes place right after II and being a bit more polished might be a bit more likely to hook someone who isn't really sure of the series, although Seven is also a good game. VIII and VI both have mentions of Memories (but not Oath because that was once Ys III).
Full story order is: Origin, I, II, Memories, Oath, (V), VIII, VI, Seven
Publication date is a bit difficult (and XSEED did some extra work when translating on some) but counting Memories and Oath as the remake dates in Japan the order is: I, II, (V), VI, Oath, Origin, Seven, Memories, VIII
IMO, VI and Memories have the nicest background art.
Panaias: Now that I took a look at screenshots I remembered 100% it was "Ark..." that I quit before mid-game due to not being able to progress. This is the one I played right after completing I & II.
I don't remember too much about that one other than that the game wanted you to kill the cutest little creatures right at the beginning :(. I found it less difficult than Origin, although possibly just because I had completed Origin by then, which took me a few tries (giving up for a few years after one). Origin also has an improved help system that is another good reason to do that one earlier, although on the potentially negative side it makes you play through the whole game three times to get the full story. I tried a second run with Hugo but didn't make it too far before getting stuck again even after beating the final boss five times with Yunica due to a really annoying bug that caused the game to crash right after you defeat the final boss (if you start getting random crashes later in that game you might want to check the forum for the solution before getting that far :/, although at least my frustration was limited by amazement that I could do it). So I went on to I, II, Memories and got stuck on Oath and eventually went back and finished Origin, then I and II again, then Oath on easy, VI, Seven, Memories again, and VIII (that I didn't realize came earlier in story sequence).
Anyway, I agree that Origin, Oath, and VI are on a very different difficulty level than the others and some people will prefer to skip them. I really wish they had a separate boss difficulty level since that was the part I most found unreasonably difficult. But, if I can do it I'm sure you can too if you keep at it. Make sure to sleep well after playing to help learn the movements :). The only question is if you would have more fun spending the time on something else.
Oh yeah, I remember VI had some items that were quite difficult to reach. Even with a walkthrough in the worst cases (optional stuff), although IIRC there was one case where I didn't realize something was reachable and thought it would be reachable later but IIRC it turned out to be necessary and reachable by a different way than I expected.