The most consistently good adapters I have been buying and using recently are the K&F Concept ones. I do not have one for a Leica M (I still have an older adapter which predates the K and F ones coming on the market). It is OK but I will update it - the K and F ones tend to be a little bit dearer than the unbranded ones but nowhere near so expensive as Metabones, Fotodiox etc ones which tend to be at the top of the range.
Whether or not you buy a K and F Concept adapter, the thing to look for with M adapters is this.......if you look inside the adapter you should find a small metal leaf spring which acts to bear against the lens' mount to hold it steady and firm when mounted on the adapter - thereby reducing or eliminating play. The better adapters have this feature. The cheaper adapters tend not to- they just have a slit in the metal of the adapter ring and this slit acts as a kind of cheap spring substitute. If this comes loose (as mine did) you can temporarily remedy it by inserting a blade to lever it open to make the slit a little wider which increases pressure on the lens mount when the lens is installed in the adapter. But I am convinced that a proper spring works better (and for longer) and if you select an M adapter of any brand which has this feature it will serve you better in the long run. My cheap one works OK and holds the lens steady now that I have levered the slit open more.
The spring (on a K&F Concept branded adapter)
The slit on a cheaper unbranded adapter