You asked
Best budget lens for the canon eos 6d?
For the 6D, I'd start with the Canon EF 50mm f/1.8 STM. It's the cheap, fast prime that Your first camera recommends for people and low light, and it's a sensible first lens on a full frame body.
If you want one lens for most things, look at the Canon EF 24-105mm f/3.5-5.6 IS STM. The 6D has no stabilisation in the body, so the built-in IS helps with slow shutter speeds.
For something lighter and wider, the Canon EF 40mm f/2.8 STM is a small everyday standard lens.
Don't skimp on glass to save money on the body. If you're buying a used 6D, budget for a new battery too, as covered in Buying used.
Written by Claude from this site's pages.