After using so many lenses, my favourite for street is the 40mm/f1.4.
Why?
Personally I prefer wide, but shooting wide means that I have to stick very close to my subject, often 1 to 2m, if using 21mm or 15mm. With strangers, it's kind of imposing to them. The natural stranger distance is 3m. If you try to get close to a stranger at less than 3m, that means you are going to have a conversation with them, hence, a comfortable distance to shoot them is about 5m away without them feeling "imposing".
If shooting people that I know, then I would naturally go for as wide as possible, 15mm/f4.5 or 21mm/f4, or even the 12mm/f5.6. During weddings and events, people tend to relax and let you take as much as possible. Using a RF with 15mm is easier because it's less imposing on the subjects. Try sticking that 17-35/f2.8 Nikon onto their nose!
The 40mm/f1.4 is a great travelling lens. It is not too wide on the street, allowing you to focus on the subject, yet letting you to get enough of the background as bokeh. If it is a 50mm, then it focus too much on the subject and not letting you enough of the bokeh. And yet it is comfortable to shoot at about 5 to 10m without people even notice. Take a look at this:
which was taken about 10m. The subject is at a very natural state. And I captured the mood I wanted during the early spring at the park.
This was taken at around 10pm, pretty dark, but I still managed to capture at about 1/15 on the 400 film.
And this one my favourite,
About 4 to 5m away, subject didn't notice I was there and still able to capture enough of her expression.
Hence, the 40mm is my favourite street photography lens.
Having said that, 40mm is not ideal for scenery. If you are on a trip to Jiu Zai Gou or any of the China scenic places, then the 28mm/f3.5 is ideal. Great lens for outdoors, sharp and contrasty. Exactly the feel you would need for a scenery.