Leica-branded adapters are the most desirable, as one can be confident of the correct thickness, which is critical, and overall quality.
Just about as good are the Voigtlander branded ones, available from CameraQuest. The Voigtlander comes in Type I with the traditional cutout in the flange, and Type II without the cutout, to cover the coding area for digital M cameras.
Some cheap adapters are inexplicably made too thick, not allowing mounted lenses to focus to infinity. Something to watch for...