From 50+ of M42 lenses I had, I would recommend the following:
East German M42:
Flektogon 20mm/f4 - Top landscape lens that is quite sharp stopped down. Many claims it has no distortion but that's not entirely true. My fav landscape lens after Distagon 28mm/f2.8.
Flektogon 35mm/f2.5 MC - Very sharp in the middle even wide open but not so at the edges unless stopped down. Close focusing (20cm?) is very handy. I would rate this is one of the best East German lenses. Single coated 35mm/f2.8 version flares alot and interfere with EOS mirror.
Pancolar 50mm/F1.8 MC - I like the colours it randers. Quite sharp wide open. Single coated 'zebra' is as good. Unfortunately the Pancolars will interfere with EOS mirror.
Sonnar 135mm/F3.5 MC - Produces smoothest bokeh. Bitingly sharp. It gets a fair share of votes for the best manual focus 135mm lens.
Jupiter 9 85mm/f2 - You love it or you hate it. The swirling and fuzzy bokeh can be distracting. Only useful stopping down to f5.6 and beyond.
Tair 11A 135mm/f2.8 - I think this is the sharpest Russian lens ever made. Good bokeh as well.
Jupiter 21M 200mm/F4 - Quite sharp wide open. Buttery bokeh. Very bulky though.
Pantax Takumars are generally very good. However, the 'favorite' 50mm/f1.4 will interfere with EOS mirror.