The MK 21 might be my favorite capsule at this point; in the situations where I can get away with using it, I know it's going to please the listeners. Of course, no one type of capsule or microphone is always best for all recording situations, and maybe one reason that I like the MK 21 so much is that I happen to like the kind of acoustics in which I can use it.
Also, begging someone's pardon I'm sure, but the "ORTF" stereo recording method is defined in a very specific way. It's not the American (or for that matter, Wiccan) ethic of "An it harm none, do what ye will." If you use any microphones other than small-diaphragm, single-diaphragm, front-incident cardioid condensers, or any spacing and angle other than 17 cm and 110 degrees, then it's not ORTF. It's not me being the purist here; the ORTF tried many approaches, and the one they came up with is the one that bears their name.
The MK 21 is called a "wide cardioid" for lack of a better name, but a "wide cardioid" isn't a cardioid. In any case, a setup with 17 cm spacing and 110 degrees between axes with wide cardioids doesn't image as clearly, or capture spaciousness as well, as it would if you spread the capsules somewhat farther apart.
--best regards