I’ve owned the Tamron 45 f1.8. It is a very good lens. Sharp, nice bokeh and it makes your subjects stand out with nice background separation. Focusing with the center AF point on my 1DXII was great, but the outer focusing points was back focusing. On a 6D, I would believe that it will do very well.
The 50 f1.8 STM is a bargain. Small and light, focus is good but a little slow. Sharp and ok bokeh. Not as good as the Tamron.
The 50L has some issues with CA, and isn’t that sharp wide open, but it takes fantastic looking images, especially when stopped down to f2-f4. My copy focuses better than any other 50-ish lens I have owned, but that is with a 1DXII and 5DIV, but still AF isn’t as good as the other L primes. Focus and recompose (which you often will do when using a 6D) will not work well. The 50L doesn’t have a flat field of focus.
The 40 STM is slow in aperture and focus. I tried it but didn’t really like it. Colors and contrast is a bit meh.
If you plan on keeping your 6D for a while, I recommend the Tamron 45. If you plan on upgrading to a 5DIV anytime soon, go for the 50L, or the 50 f1.8 STM if you have better things to spend your money on.