For street photography you generally want a slightly wide to moderately wide lens that's small and unobtrusive.
Using a crop camera (or FF, I'd recommend an EF 28/1.8 as a good possibility. It's got a reasonably large aperture, USM focus, and is quite compact, even with its matching lens hood. The 24/2.8 (non-IS/USM) would be another possibility, except it's f2.8 and doesn't have USM. 24/2.8 IS USM is a lot more expensive and a bit larger than the non-IS/USM version.
There aren't any particularly small/compact 21 or 20mm lenses, unless you consider something vintage with manual focus and exposure control only. Then you might look for some older Olympus OM or Nikkor AI-S/F, which are easily adapted for use on Canon. There are some f2.8 versions and they can be reasonably compact, but aren't cheap.
Part of what's complicating this is also wanting to later use the lens on FF camera. A vintage 17mm (Tamron or Tokina), or a modern Rokinon/Samyang/Bower 14mm might serve for street photography on a crop camera, and are usable on a FF camera, but there they would be ultrawides and wouldn't serve the same purpose.