Bruce,
I have examples of all three. Although none (well, maybe one) are target-grade barrels, and I'm not a competitive target-grade shot, here's my experience for what it's worth.
Of course, the .25-20 repeater is easier to find components for, and that's a big advantage for some. It's plenty accurate for small-game and short-range varmint shooting or plinking. I can typically get 2-1/2" groups at 100 yards with it with iron sights and cast bullets. I found it took some doing to get groups with reloads as consistent as those with the lead-bullet factory ammunition I tried, although eventually I was able to do so.
The .25-20 SS seems (to me, at least) to be easier to load to greater accuracy with canister powders than the repeater version. I can get under 2" at 100 yards with this caliber, if I'm doing well. Also people like Allyn Tedmon and others in the 1930's were able to load the .25-20SS up till it was a sort of large version of the .22 Hornet, in strong rifles, while everybody agreed that the more extreme bottleneck of the Repeater version raised pressures too quickly for such loadings. I've tried loads along this line with my Win SS in .25-20 SS (with bushed firing pin) and found that the groups I was getting, except with the obsolete Winchester 60-gr HP bullet, weren't accurate enough to take advantage of the enhanced (2600 ft/sec range) velocity. I'm usually doing good to get 3" groups at 100 yards with the modern Hornady 60 gr, although the Win 60 might cut a half-inch off that. Still, the potential is there. The old guys used the Win 60 grain exclusively, which was all they had. I look for boxes of that bullet at every gun show I attend, generally to no avail.
The old experimenters all held the .25-21 to be the most accurate of the three, but the black-powder propellant they mostly used might have influenced this judgement quite a bit. Supposedly you could get 3/4" groups at 75 yards with this shell, loaded with black powder. Townsend Whelen had a Win SS in .25-20 SS bushed and rechambered to .25-21 and wrote that he got much more accuracy out of it afterward. My own experiments with this caliber are kind of inconclusive. Sometimes it shows more accuracy than the other calibers, then the next weekend it doesn't. This may be due to smokeless powder, and may have a lot to do with the Bertram cases I use, which seem to vary quite a bit in length and even in diameter, not to say rim size.
I have three rifles in .25-20 SS, one Win Low-Wall, and one barrel for a Stevens 44-1/2 and another for a Stevens 47/44-1/2. My .25-21 and one .25-20 Repeater are barrels for the Stevens 44-1/2. I also have a set-back and rechambered Stevens 44 in .25-20 Repeater. (I have a Marlin 27 pump in .25-20R as well, but the bore is pretty bad, so I don't include it in this.)
So I would say if you don't mind the extra expense and hassle, the .25-20 SS offers the most versatility. If convenience is the most important, the .25-20 Repeater will give you 95% of everything else you want. If you are a total crank, get the .25-21, experiment every which way and weep whenever one of the Bertram cases splits
.