35mm prime
$$$35mm (24mm on APS-C) is wide enough for a street corner and tight enough for a person. It is quieter and smaller than a zoom, which is the whole point. f/1.8 is enough; f/1.4 is weight you will feel by dinner.
What to look for: 35mm full-frame equivalent, f/2 or faster, compact. Autofocus is nice; zone-focusing with a small prime still works.
Skip it if: you already like your 28–70 kit zoom and you actually carry it. A 35mm is a size-and-discretion upgrade, not a requirement.