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'Best' is a very relative term here. The Planar 50/1.4 is a very good lens, very sharp stopped down, with decent bokeh and great color and contrast. However, the Planar 50/1.7 is sharper wide open (and perhaps even stopped down, but you'd be splitting hairs there), but has rougher bokeh.
A 40mm lens on the speedbooster is hard, as there isn't much, and you're limited by mounts now. Minolta has a good 40mm pancake, but it won't work on the speedbooster, since there isn't an MD booster and it's got a shallower mount so you can't do EF with another...Show more →
There are three other excellent options for 40mm's without any modifications for an EF Speedbooster, and an additional one if you are willing to permanently modify the lens. The OM 40 and Voigtlander 40/2 Ultron are both excellent lenses and suitable for EF Speedboosters (the CV is native on EF or adaptable from nikon F), additionally you have the Canon 40/2.8 STM which by all reports is quite a lovely little lens, but slower and if you are willing to apply a leverectomy there is also the Pentax SMC-M 40/2.8. At 45mm you get the C/Y 45/2.8 Tessar as well as the Nikkor GN 45/2.8 and the newer AI-P 45/2.8, both of which are Tessar clones. The Tessars are sharp in the centre wide open, and have lovely bokeh but need to stop down to get sharp at the edges.
Did Minolta even do a 40? They did do a 45/2 semi-pancake which is a lovely (and dirt cheap) lens.