The best ever focusing screen for Pentax crop-DSLRs is a Nikon F6-A focusing screen. Actually, it was the best screen, I must confess that I bought the last one about a year ago
Usually, one wants to replace a focusing screen for more accurate focusing. In my experience it's only possible when a screen has a split-image sector. Plain matting will never be as good as split-image for accurate focusing.
However, the problem with split-image screens for crop-DSLRs is that split-image sectors are too big, they distract from composition and can and do shift exposure. It is because precise screens for crop-cameras are made from FF screens. On FF cameras such sectors look not very big but when screens are cut for crop-cameras, everything on screens is magnified.
Nikon F6-A is the only high precision screen that has a very tiny split-image sector and no other distracting elements. Once installed in a Pentax DSLR, one can't find any distractions at all except for a rather small split-mage sector in the center of the image. There are no exposure shifts at all. And the focusing is very-very accurate!
After F6-A the only option is Canon's EE-S but it's only a subtitute.
P.S. I sold my K-3 but I'm still keeping the F6-A screen. If someone needs it, you can send me a PM. I also have a Canon EE-S screen for Pentax, F6-A is much better for sure.