I haven't used many 50mm lenses. I have used a couple of Minolta 50mm f/1.7 lenses and they are not very good. I used about 3 Nikkor 50mm f/1.4 lenses and they are fine. But both the Pentax K 50mm f/1.8 and 50mm f/1.4 lenses really impressed me. The Pentax 50mm f/2.0 is not very good.
I've used Pentax f1.4. f1.8 and f2 50/55mm lenses and they are all superb that's screw mount as well as K and M series, there's no drop in quality with the f2's.