I have a problem with absolutes in creative pursuits. There are answers that are more common, but everyone has to answer this sort of question for themselves. That takes time and lots of effort.
My answer is 35mm, but I can understand how a 50 would do it - just enough to isolate a subject, not too much to lose the environment. I like having a touch more of the surrounding area in, so a 35 feels right for me.
Dogma is bad for your karma.