Now this a tough question. Lemme check...
The M 20mm f/4 doesn't see much use these days because it is not wide enough on APS-C. In my film days I found that it was just a tad too wide and required some thought to get good images.
The M 28mm f/2.8 was wonderful with slide film. When combined with a polarizer, super saturated colors was up for grabs. On crop frame, it is a bit of an odd thing. Not wide and not quite a standard lens (close though).
The M 50mm f/1.4 is great for shallow depth of field. I have only used it on digital APS-C bodies. Getting the focus right is tricky for this old timer. Live view helps. A film era focusing screen screen would be useful too.
The A 50mm f/2.8 Macro is just wonderful on crop frame and full frame. Super sharp and easy to use in full program mode. Just enough DOF wide open to make focus easier than the f/1.4.
The M 100mm f/2.8 was a favorite in the film era. It is still nice and I should use it more often. I do yearn for the bokeh madness that a modern 85mm f/1.4 can do, but that is another story.
The K 200mm f/2.5 is quite the shallow DOF bokeh monster when you get it right. Sometimes you get an almost 3D feel to an image. Not easy to focus. The same caveat for the 50mm f/1.4 applies, only more so. Also a lens that I have only used on crop frame.
The M 200mm f/4 is another long time favorite. Quite sharp and easier to focus than the f/2.5.
The A 400mm f/5.6 made proper sport photography possible when I bought it new in the late eighties. It still works well for that purpose, as well as bird and wild life photography when I get around to it.
My favorite? Hard to say, but I will go with the 200mm f/4. It was my first Pentax lens that I bought new and is probably the one that I have used the most over all the years.