I voted B+W. But in reality, I use several brands. I do favor B+W for the MRC coating, which is the next thing to having no glass at all. But the B+W filter rings are very thick, which can be a problem with wide-angle lenses. Heliopan, on the other hand, makes a very thin filter ring. I don't believe anyone even mentioned Heliopan. You know, they had the ad with a moose hiding in the bushes, and he says, "I'm not coming out unless it's a Heliopan filter?" Nikon makes thin filter rings, too; but hey, this is a Leica thread, and Leica doesn't use 52mm filters.
Hoya? One of the sharpest pictures I ever took was through a Hoya filter. They have several different coatings at various prices. I believe the best is the HMC, or Hoya MultiCoated. It is a good brand.
Tiffen. Have you ever seen a coated Tiffen filter? I have not, and I have a bunch of them. When I hold up a Tiffen, all I see reflected is the color of the light bulb I'm using for illumination. No blue, green, or red reflections. I don't think they coat them. Too bad, since they make most of the color correction filters I use, like 81A, B, and C, and the Tiffen 812, which is kind of double-strength skylight. I use 'em when I need 'em, even though not coated.