In my film days, my go-to focal length for street and candid shooting was 35mm, without exception. When I went to digital, choices became more divers, mostly because there are so many affordable good lenses. So far, I've tried 24mm, 28mm, 35mm again, 40mm, 50mm, 75mm and 90mm - all equivalents as of yet since I don't own FF.
24mm was/is too wide for me - I just don't see that way and can't seem to balance street shots. 28mm (the Ricoh GR!) sometimes works very well, but I'm still not able to adapt to the FOV all the time - but from the results I'm getting, I think it's one of the most useful focal lengths for street work. 35mm is a no-brainer from days of old, but I've actually come to like 40mm more - though I think that's mainly because I prefer the rendering of the Panasonic 20mm (I) to the also very nice, but somehow dreamier, less crisp performance the Olympus 17mm.
Interestingly (mainly in the light of what john m flores has said above), 50mm already puts me in street portrait mode - something I really like doing, but I don't know if it's still proper street photography - truth be told, I'm not into creative restrictions of that kind myself, so I don't really care, but the images I get are visually quite different from those taken with wider lenses.
With moderate tele lenses, 75mm or 90mm, street portraiture is a given - I admit that Ilove doing that, even though it's somehow less spontaneous and dynamic as an approach (pun intended).
Finally, I went out today with a fish-eye (15mm equivalent). Crazy stuff - but some of those images have something I need to explore more. Strangely, I like the fact that there are no straight lines - imperfections can be liberating.
Bottom line? Don't limit yourself, at least not before exploring the possibilities available to you. Find a style (or several) that suits you and/or your equipment. For me, there's something interesting to every setup. There's certainly no single best focal length and/or lens, but that has been established already.
M.