+1 on the 24L II on a full frame and the other lens I used a lot for street photos was the 135L, which is good to get more discrete shots. I now use the 24L II & 50L about 95% of the time for street photos, but I'd say any fast prime between 24 & 135mm is going to be your best bet. The 16-35/17-40 and 24-70/105 lenses work well, too, but primes are smaller, lighter, and more likely to give you "that look".
The focal length really depends on your preferences, there's no right or wrong choice. Take a look through your photos and see what focal length (in the EXIF) you've used for your favorite shots. That should help you narrow down your preference.