Except for the JML 21 mm f/3.5 in the back row, these are all lenses that either can be purchased new or come up fairly frequently on eBay.
They're roughly in order by overall image quality, worst at lower left, increasing across the front row, increasing again across the back row starting at left and ending up with highest quality at back right.
The test images were shot as stacks and processed through Zerene Stacker PMax to bring out difficulties in the corners. The target consisted of 600-grit silicon carbide sandpaper, scored with a spring steel pin so as to crush particles and produce lots of fine detail. The target is described in a separate thread, HERE.
Camera-resolution images for evaluation can be found here (in order matching the above photo):
no-name achromat, commonly found on eBay shipping out of China, very small image circle, severe aberrations on sides and corners of APS sensor at 4X. This objective is advertised as DIN, but it appears to actually be JIS, with a parfocal distance of only 36 mm instead of 45 mm for DIN. As a result, the working distance is shorter than you'd think also. For our purposes, avoid this one.
Nikon E achromat. This old objective appears to be in the CF line, but it actually has obvious color fringes. It's also not very sharp in the corners. Not shown above but tested separately, the Nikon Finite Conjugate 4X sold by Edmund Optics is very similar. Avoid both of these.
AmScope plan achromat, optically similar to the Cnscope, both lenses have differences from one corner to another. With both lenses corners are softer than the EL Nikkor but the center looks better.
Cnscope Plan Achromatic, covers most of the frame with high quality image. If you need an inexpensive DIN format objective, this is well worth the price at $25. What appears to be this same lens is widely advertised on eBay shipping out of China, not always advertised as "Cnscope".
EL Nikkor 50 mm f/2.8 at f/4, more uniform across the field than any of the small objectives, but not as sharp in the middle. At typically low prices, definitely a Good Buy.
Olympus 38 mm at f/2.8. Somewhat sharper than the EL Nikkor, with more uniform corners.
JML 21 mm f/3.5, similar sharpness to the Olympus and the absence of false colors is striking. An outstanding lens, if you can snag one.
MPE-65 at f/2.8 and 4X, slightly sharper than all except the Nikon CFI combo, better uniformity especially in corners.
MP-E 65 at f/2.8 and 5X. For direct comparison with the CFI combo, next.
Nikon CFI 10X Plan Achromat on Canon 100 mm macro lens, giving 5X. This combo is much sharper than the MP-E 65, not surprising since the MP-E is running around f/16 effective while the combo is about f/10. What is surprising is that the combo holds up well clear to the corners, despite that this far exceeds the field size that the objective was designed to handle.
Comparing up and down the line, there's really not much difference in central resolution until you get up to that last combo.
Edges and corners are a different story. The small fields of the finite objectives are a bit of concern for use on bellows, since any misalignment of the front standard will shift the optical center back on the sensor. For critical work it'd probably be a good idea to check that.
I hope this is helpful.
--Rik
Edit: to add test results for the Nikon Finite Conjugate 4X sold by Edmund Optics.
Edit: to add link to target description
Where does that non-name Chinese 4x Plan Achromat fit into the list?
vs 
It's interesting that it is so close to the EL-Nikkor but they have complimentary faults ... the EL-Nik being not quite as sharp in the middle but the Cnscope is worse on the edges. I think I will try a Cnscope because that central sharpness will be useful for what I am doing 
