I think there is more to it than just whether the adapter focuses to infinity (an undoubted necessity) but when other things are much the same the following can apply:
(This dialogue assumes that if they fit camera to lens then they work. I have not seriously tested for infinity focus but on the other hand I have not noticed any particular problem in this regard either).
Level of finish A = average or agricultural, a bit tractor-like, or G = well machined and workmanlike, or E = excellent and really graces the camera
Weight - some can be made like bricks - H = heavy, or R = reasonably compact
Fit - smooth and tight - some can be clunky or loose - S = smooth, or C = clunky
Does it work as it is supposed to do? W = works, or N = not suitable
Does it have additional features? - surprisingly some adapters do. AS = aperture lever setting, or AL = can be re-aligned to match the lens barrel markings.
How reliable is the vendor? No comment unless this is notable.
Is it particularly expensive or particularly cheap? No comment unless notable.
I have included both NX and LM mounts in this survery of suppliers as if they can make one adapter they can make many.
Those made of composite camera lens-barrel like material external finish will always look better on camera than machined and anodised aluminium construction.
RJ (Rongjin) - manufacturer Shanghai - many mounts serviced - composite construction allows AL and (AS in FD only) Good looking, can hardly tell that they are mounted due to such similar material being used. R,S - price reasonable considering quality. Some effort made to keep adapter external width within lens end diameter.
Cierco - ex-Poland - custom made and expensive but well made - exquisite machining of FD mount internals, good fit, anodised black, single piece. Good service - has a range of adapters for almost every concievable rare mount combination although I have not made a study of this.
Pixco, Kiwi-foto - similar (same manufacturer?) ex Chinese retailer - common. The ones I have are single piece anodised black aluminim completely dumb and with no adjustments. Well made and work well. Good fit on lens and on camera. There are some other composite-made Chinese made adapters that are fairly wide in diameter that work well enough but are larger than lens end and therefore not so sleek fitting - they have zebra-stripe markings - not branded on the adapter and I don't remember the brand (might be Pixco).
The Japanese Zykkor EF/FD is exquisitely made with inbuilt conversion lens and so is the oem Samsung PK-NX, the best one I have ever seen complete with aperture control slider ring and priced accordingly. If made for Samsung by a Japanese or Korean company it is a tribute to them - otherwise a tribute to Samsung.
A Russian made LTM-NX from Gofoto Store and a Latvian sourced M42-LM are single piece (non-adjustable in M42) hard black anodised aluminium and work well, good fit and machining and acceptable. The LTM-NX is small and hardly can be seen on camera and probably the work of a very ingenious (for the NX fit) small shop. There is a Chinese version again made by a small shop in two piece chromed metal. More complex construction but in the case of that adapter I prefer the more straight-forward Russian design. This is not so much help with LM adapters as a general survey of suppliers. Small shops can cover the more rare situations at a closer to jobbing price structure.
I have a second hand "rare bird" Canon "B" oem FD/LTM adapter second hand with excellent shipping from Spain. It needs a LTM/LM converter to work, but is high-quality thinwall bright aluminium construction. The FD mounting lugs are external. It seems optimally made to be compact and light. If I had to keep the size of my adapter collection in the gadget back minimal this adapter and the normal LTM/LM is surely the best way to go. There is no reason why the LTM/LM could not be left attached permanently.
The Novoflex adapter I have is a disappointment. Works really well, nicely machined, has an aperture adjusting lever (is PK/NX) but is anodised "Novoflex Blue" to make a statement and as far as looks it is more tractor than sports car.
In finish the Japanese (#1) and in China "RJ" have the running.
The RJ LTM/LM adapter works fine but mounting is more problematical than the Voigtlander version. The RJ has to be "just so" square before it will slot home. A clumsy touch might damage the camera mount side. It is the only adapter whose mounting ability I would clasisfy as "clunky". I know about it and am careful. But the Voigtlander version slots in just that much easier. Have not tried any others but there are many to be had.
The only real failure has been "Antonio Lolli" COMA brand from Italy. Their PK/LM works fine and is adjustable, but the adjustment screws are not flush fitting. It is of heavier construction and must be described as "heavy build". But it is not overly expensive and works (as noted). On the other hand, just as much as the Canon "B" is the essence of sweet lightweight elegant simplicity the COMA FD/LM is a tank but only by direct comparison. It has no built in actuating lug to allow the aperture to be changed when the lens is mounted. This is a shared defect with the Canon "B". The "solution" is to jam a little piece of rubber in the lever mechanism (similar to how the PK lever works) to hold it open - this usually falls out inside the camera body - not good enough for those who respect what goes inside their cameras. After some exchanged debate with COMA I simly gave up and retired the FD adapter to the scrap heap. I had meanwhile acquired an RJ PK and FD adapter to LM and the COMA adapter quickly became surplus to requirements.
Both the RJ and Cierco FD adapters have an internal lug that works this lever - much better.
Probably missed some.
However the message is that by and large all these adapter work well enough. The COMA FD alone falls below what I might consider usable standards. The Canon "B" is a very old design and rare and worth having as such - it is at about COMA FD servicability but at a fraction of size and weight. An enterprising person with some technical skills might be able to self-fit an operating lug inside. No way that a similar operation could be performed on the COMA - it's design is not suitable for this. The RJ LTM/LM reuires a little concentration in fitting being perhaps a fracion over-precice in its lugs but otherwise it works fine.
So if they all work, then it gets down to looks, price, finish, etc. The Japanese brands seem to have the best finish, but RJ are not far behind. The RJ M42 adapters have a neat composite design that allows the lens barrel markings to be adjusted. Their FD adapter has an aperture adjusting rotatable outer ring which is hardly necessary but it makes hooking up the FD's awkward internal aperture lever to the lug a little easier and also performs an aperture preset function just like some of the older lenses pre "jumping diaphragm".
Most other Chinese, FSU and Polish sourced adapters are well made and look practical but are not made to look like an extension of the lens fitted. The Novoflex is really very well made but somehow seems to think that their blue anodised is some sort of status symbol. Maybe it is but perhaps not on a lens adpater. I do believe that they make some adapters in regular black finish though.
My short experience with Voiglander adapters is just their LTM/LM which is impeccable.
Seems like in general terms if you need "royalty" to attach lens to your camera then crown jewels prices from a select manufacturer are required. But if you need one that simply works then there are a wide range of suppliers who make a servicable product.
My favourite has become RJ but only COMA (in its FD only) has really disappointed. I forgive the Canon "B" simply because it is a rare beast such and an elegently engineered tiny little beauty.