^ Interesting, I had both the 6D and 5D3 and really didn't notice any difference in sharpness between the two cameras. The more reliable AF of the latter gives that camera the edge IMHO. However, I do notice a difference between the 5D3 and 5D4 (latter is a bit softer).
I'd be interested to see how the 6D2 compares to the 5D4 IQ-wise.
I'd count on the 5DIV for action shots, wildlife. I'm a little disappointed by the IQ at ISO 400 and above, just not as clear and sharp as hoped, not as smooth as the 5DIII.
While I generally concur with this sentiment, I'd also note that at
really high ISO (6400+), the 5D4 has NOTICEABLY less color noise than the 5D3 does (and the 5DSR too by extension). This leads to smoother, more useable files and slightly richer colors. Having had to shoot both cameras at ISO 10,000 for select events, the difference was quite clear in the amount of work the files needed in post.
Definitely mixed feelings regarding the IQ for sure.