Tony Northrup recently has made a video comparison on image quality and dynamic range for recently released Nikon D850 with D810, 5DS R, a7RII, E-M1 Mark II. The Nikon D850 is now already shipping, you can get at Amazon/B&H/Adorama/Wex Photo/Jessops. Also check in stock status of Nikon D850 here.
From the video comparison above, we can clearly see Nikon D850 is much better than D810 and 5DS R in both image quality and dynamic range. While compared with Sony a7RII, it seems there is not much differences in both image quality and DR.