I own the 1DX II and had a good chance to test the high-ISO noise performance last year in Patagonia. This shot was taken at ISO 8000, for example (with noise reduction applied in post):



I have found the Mark II is a very slight improvement over the Mark I that owned previously. Maybe half a stop better? I also have the 5D Mark IV and it is almost as good as the 1DX II. The noise is slightly more obvious but it is very fine-grained, probably owing to the high megapixel count. To me the raw image noise is almost irrelevant --- what really matters is the quality of the image after processing. When you remove noise, the picture will inevitably lose sharpness. So a camera that has more megapixels is likely to be noisier, but maybe it can also withstand a higher degree of noise reduction before the image quality suffers noticeably (please correct me if I'm wrong). So which camera gives the best post-processed image? I'm not actually sure. But I'm definitely happy with the 1DX II.