OM4: OM Zuiko 50/1.4 (I wouldn't mind trying the 50/1.2)
Barnack: probably the collapsible Summicron 5cm but the J8 is also very good, also the Canon 50/1.8 LTM. If I want sharpness the Industar 61 is hard to beat.
Topcon RE Super: Topcor 58/1.4
Nikon F: early Nikkor 5cm f/2 with nine aperture blades
Pentax K: Pentax-M 50/1.4 but I haven't any others to compare. If I had M42 I'd go for the 8 element 50/1.4
Canon: I prefer the EF40/2.8 pancake STM to the EF50/1.4. Kostya's 50L sounds better.
Fixed: the R35S only comes with a 40/2.8 bit it's a beauty. The Yashinon 45/1.7 on the Electro 35 is a great lens but I and the camera don't share any karma.