Leitz and Voigtlander adapters are the ones that are always on spec.
I tried a fotodiox adapter once. Such a crude tight fit that I was afraid that it would never come off my M7. The machining was clearly out of spec.
The Russian adapters that come with the Lomography lenses are also quite good. They don't have such a nice finish as the Leica or Voigtlander adapters though.