I'd suggest the FL 55 f/1.2 with an EdMika adapter, probably $3-400 all up, but it doesn't work on a 5D2, but does work on the 1Ds-series. Remains to be seen if it'll work on the 5D3 or 1DX.
My Asahi Pentax Takumar 50 f/1.4 is also a great fast normal prime, $100 or less shipped, they're fairly common. Great colours and sharp as all hell after f/2.
But both of them are MF-only, and the 5D3 doesn't have changeable focussing screens. If you're shooting street you can always f/8 and Hyperfocal, set it to M on 1/100 or so and auto-iso, the 5D3 looks clean up to iso 6k or so, you should get great images that way.
Or with Autofocuses, it's the Nifty Fifty f/1.8 II or 50 f/1.4, the 1.8 is sharp stopped down but built like a toy, the 1.4 is badly in need of an upgrade which should happen this year (which we've all been saying for the last 5 years). Never used the Sigmas, from what I hear they're sharper than the canons, but the autofocus is iffy...