Isn't technology grand?
Of the lot, the D-171 is my favorite just because I've had it for so long and its reliability after all these years, but I have an real affection for the D-10. While not being the original Discman model I owned, it was the player that—somehow, considering how much it skips if you sneeze in its general direction—got me through years of commutes in San Francisco; an undeniable nostalgia factor. The sound quality itself is also so much better than the other two there's just no comparison and for that reason alone it's the one I listen to the most. I just wish it played all disks as reliably as the other two. I'm not even talking about CDRs (surprisingly all three will play them even though CDRs weren't even a thing when the D-10 was in production). For example, the D-10 absolutely refuses to play all the way through Revolver, the last track on disk 1 of Madonna's Celebration (an absolutely pristine commercial disk, btw) and no amount of cajoling can convince it to do otherwise. (The disk plays flawlessly everywhere else.)
But admittedly I do like the quirky design of the D-EJ100. You can't see the disk spinning like with the others, but that neon lime green center circle display is an absolute chef's kiss in my opinion. Of the three, it also has the best skip protection and fits inside my headphone case like it was made for it, making it the perfect traveling companion in the already overstuffed messenger bag haul back and forth to the office. The sound quality isn't as good as the D-10, but it's good enough to get though those three days I have to be physically present at corporate HQ.