Careful - these things can take on a life of their own
Kernos' suggestion to look at photos and decide what you like is a good one.
If you want macro, the Macro Tak 50/4 is a nice lens. There’s also Vivitar 100/105 variants. I have the Vivitar 100/2.8 Macro (1:1) and the 135/2.8 (1:2) and really like both. Tons of others but those are the ones I can speak for.
The Mamiya-Sekor 55/1.4 and 58/1.7 are among my favorite 50ish lenses for their sharpness, colors, and bokeh.
Helios 44-2 58/2 is cheap ((but getting more expensive) and sharp with really cool bokeh. The Helios 40-2 85/1.5 is my next M42 target but it is spendy.
In the 50-58mm range there are tons of great lenses and it comes down to rendering subtleties or tactile/cosmetic preferences. Outside that range you need to be more selective (just my opinion).