Sports, high ISO, video - 1DX mkII
The 5D MkIV has same AF system as 1DX mkII but at slower frame rate at only 7 fps. I prefer having the option of removing the battery grip and going "light" with my 5D and having the larger sensor. It's a great all-around camera.
However, if your primary purpose is sports action, then go with the 1DX mkII. Here is a shot of a race with my 5d mkIV. The better shot would have been the moment both lead dogs had their heads up at the top of the stride, which would have been captured with a 1DX mkII. As it was, my buffer was filling from the previous burst and I was surprised to get this one.