I love the 23mm focal length, but I'd say it's a toss up depending on what your personal preferences are. 35mm could be an excellent option as well. Either of those two will probably be an ideal prime to complement the zoom. I probably do 90% of my photography with either the 23 or 35 and the remainder is a mix of the 16mm and 55-200mm. The 23mm and 35mm may seem fairly close in focal length, but there is a huge difference between them when you're actually out shooting. I'd get as close to each of those as possible on your zoom and try shooting at only one focal length for a bit. Try at 23 first, then 35. You very well may end up owning both of those primes eventually, but that will help you decide which to buy first. The other question is how comfortable are you getting close to people in your street photography? If you're very comfortable with that, then the 23 makes a lot of sense. If you're not that comfortable with getting close to people, the 35 might help then you can get the 23 as you get more comfortable.