This is probably the hardest question to answer, because there is hardly a more refined focal length in 35mm lenses - its that one lens that at its worst is still very good (as has already been mentioned). Because of that, I tend to agree with Dave that "Best is subjective at Best". Aside from phenomenon such as the "Leica glow" and other highly questionable claims, I look at these lenses like so: the budget brands that usually are a little more flare prone and show some of that angular bokeh (apparently due to the number of apperture blades), and everything from 1.4 (usually - but not exclusively - this is the threshold, please keep in mind exceptions of high end, lower speed lenses) up. In a special place are the things like the Nocti-Nikkor that is specifically corrected for night photos, the legendary Noctilux, and the Canon 1.2 SSC ASP, which apparently outperforms them both... On the other hand is the argument that those lenses are so optimized for wide open, high contrats night work, that they are not nearly as good for general photography as their less exotic cousins...
One could go on, and on, and on...
But, perhaps a more productive approach would be to ask:
"What is YOUR favourite 50mm, and WHY?"
That way we could see the little individual traits that endear one lens to one person for one reason, while keeping in mind that these are preferences not be all and end all judgements based on miniscule differences.
Peter.