There's no doubt in my mind that the D810 is the best "all-round" DSLR ever, but it also excels in "niches" such as my personal favourite -- wild birds in habitat. The combo of the D810 coupled with a pro-quality lens is nothing less than awesome.
This image of a Great Horned Owlette is an almost full-frame image (0.9 linear horizontal crop) taken with my D810+600E+TC14EIII, 850mm, 1/160, f5.6, ISO 400. Note: this was shot in evening light, hand held wide open at a ridiculously slow shutter speed for the focal length, which says as much about the capability of Nikon lens performance as it does about the D810. I took more than a dozen images at different poses and angles, and all but two were tack sharp.
This image has been reduced to 1600x900 for posting here, but if you're interested head on over to my flickr page https://www.flickr.com/photos/garyirwin/ where you can download and pixel peep the full-res 6771x3809 image by clicking on the download icon in the bottom right-hand corner.
The D810 is one awesome camera that still can do things no other body can.