In December 2012 I spent a week in Tokyo, decided on this trip to concentrate on something I'de really never tried much of, "Street Photography", I had a brand new Sigma f1.4, Canon 50f/1.2, Canon 135f/2 & the Canon 24-70f/2.8 II, all these Lenses at times were remarkable when shot on the 5DMKIII, When I looked later at the Photos taken I was unable to tell which Lens took what photo, but the Lens that impressed me most, and was the Lens used for 50% of the Tokyo Photos, was the Sigma 35f/1.4 (Strangely I've never owned the Canon 35f/1.4 so cant compare).
Although it's been mentioned that the Sigma 35 might have a slight IQ advantage I didn't find this to be the case, all these Lenses have remarkably good IQ, but the f/1.4 combined with the 35 focal length makes this almost the "Perfect" street Photography Lens I thought, albeit these are the opinions from someone that's generally more at home shooting the 600f/4 on the 1Dx and pointing at Leopards or Lions and the only people around are part of the Menu.
The least used of the 4 was the 50f/1.2 & 135f/2, again both are amazing Lenses for this sort of Photography but I often found 50mm & particularly 135 were to long as a fixed focal length, although I suspect this was more about me & my lack of experience in this "People" environment than about the Lens, usually the case.