There are a lot of good 50s on the market. However, the Nikkor 50/2 (any version, even the old Nikkor-H with the single coating - I've not tried the older Nikkor-S with seven elements though) is an excellent lens. So too is the AF 50/1.8 Nikkor. I never used a Planar but just about all of them are good.

One un-obvious choice is the 50/2 Pentax. The 1.7 is just as good. Pentax shooters should definitely own one of these lenses.

I have the 50/2 AI Nikkor and an AF Nikkor 50/1.4D and am happy with both, but the 2 is a hair better than the 1.4. The 50/2 Jupiter-8 Russian lens is pretty nice too, but in a different way...