I don't use manual focus lens as much as I used to, in large part because I have been able to find modern glass, including zoom glass, that's just better. Eight, nine years ago that wasn't the case. I can remember when I acquired the M 20 f4, I was so impressed with the lens that I wound up selling the my DA 12-24. The images from the M 20 just looked better — there was no way around it. But compared to the DFA 15-30 — that's a different story altogether. Nonetheless, I still have a soft spot for this lens. I've struggled to get good images out of it with the K-1. It's a rather fussy lens, with a narrow sweet spot. But if you throw the right kind of light at it and a subject that plays to its strengths, the results can be magical.
The manual focus lens I've used the most in recent years is the A 50 f2.8 Macro. Of all the manual focus lenses I own, it comes the closest, in terms of performance, to modern lenses. It's one of the sharpest lenses I own. But the most indispensable of my manual focus lenses is the K 50 f1.2. I'm not much of a narrow DOF or bokeh photographer, but when I want to indulge in that sort of photography, nothing really matches it — at least not at that general focal length.