As someone who lived through that era and has owned maybe 30 MD portables, my perspective is a little different.
I've owned the Sony MZ-E10 (the thin one). Not only was it very vulnerable, it also obviously had some sort of digital-to-analog artifact coming from the digital amplifier that was less of an issue with the Sharp DS8/DR7/DR80. IMO Sony would not hit their stride in digital amps until the MZ-NH1 -- that "HD" digital amp was a lot more refined.
Having heard all the digital amps in modern DAPs and smartphones, I just cannot say for sure that these old portables will beat anything hands down. There's a part of me that's always got a question mark on the possibility of those "artifacts" I mentioned. I willingly acknowledge that my memory could be very bad, but this is the dominant memory of those early digital amps for me...
The non-recording players may be more sacrifice than they're worth, especially from today's POV where even the smallest, niftiest model is not all that compact at all. Not when we've got the ipod shuffle! If you have a practical reason to want a separate unit for playback, I'd get a recorder for that purpose too. Better user interface that should in most cases allow you to forego the remote.
If you go back further in time to tank-like portables from the mid-90s, they might have very good D/A conversion or very good amp, but those advantages were meant to cover for the subpar ATRAC codec at the time. Until around 1999-2000, many MD equipments risked not having quite enough computing power to properly decode ATRAC as intended, which is why we needed to worry about ATRAC chip versions at the time. This pretty much went away after 2001,02.
The portables I find truly memorable and would want to experience again today are the Sharp MT831 (or get its Kenwood clone L7R, for a darker tone)... I'd pick the NH1 or RH1 from the Sony camp, or the R909 for an earlier model.
If I must name a non-recording player, the Panasonic MJ99 is very pretty in jet black, but sound is meh and ergonomics is as crazy as one of those real housewives reunion shows. I love the Panasonic house sound but it's no wonder why they could not sustain their portable audio business.
I have just dug out my old mini disc players I have a MZ-R900 and a MZ-NH700.
Functions wise the r900 is a lot simpler to use just lp options the nh700 is more complicated with all the net and hi md stuff.
But I was more interested in sound quality and how they compared. So I recorded a few track with them in SP mode from my music server playing flac files over a optical output.
Then I rigged them up to my amp and a pair of oppo PM3 headphone for the comparison.
The first thing I wanted to do was set the outputs to line out but its only the R900 that could do this so to be able to do a fair comparison I set both machines to headphone out turned off all bass altering settings and turned the volumes to maximum.
I did the test with various music some dance some soft rock and some piano.
The nh700 had slight more bass and treble brighter and slightly more forward sounding.
The R900 was a slightly smoother flatter sound. with a little more air between the instruments (only really noticeable on the piano track).
I would say they were both very good sounding machines. I could only tell the difference by doing a side by side comparison by switching the input back and forth.
Overall I prefered the sound from the R900 I would say it sounded slightly more in control of the music and the nh700 emphasized the bass and treble just a touch to much for my liking.
But I would be happy with the quality from both machines.
Afterwards I wanted to compare the R900 line out with my Yamaha MDX596. but I found the volume level of the yamaha was a lot louder so it was difficult to get a good comparison.