chauncey said:
Were I to list importance, noise and dynamic range performance would rank higher than would MP.
When you decide to look for a camera with "absolute lowest noise" but must choose which ISO you expect this camera is the supreme king.
In a simple way:
5DS has very low noise at ISO 100 or 400, maintaining color accuracy and sharpness compared to other models.
1DX has very low noise at ISO 3200, maintaining color accuracy and sharpness compared to other models.
Sony A7Sii has very low noise at ISO 6400, but the colors are a bit strange and sharpness is not as good, compared to other models.
Canon ME20F-SH has very low noise at ISO 25600, but the resolution is the smallest of the above models.
So I can say that Canon ME20F-SH is the champion at extremely high ISO, but that does not mean it is better than the other cameras at ISO 800. I can also conclude that 5DS is the champion at ISO 100 or 200, but that does not mean it is better than the other cameras in ISO 1600.
The amount of noise is not linear over the entire range ISO, and no camera model will be better in all ISO settings. A particular model of camera can be optimized for the best performance in low ISO, and another model for high ISO, and color characteristics and sharpness will depend on the choice that the manufacturer did when design this camera.
Canon 1DS Mark iii, was a project that prioritizes the best color and clarity with the technology available at that time, and suffers from above ISO 800. Canon used the same "philosophy" in the current 5DS, but technology now allows to reach the same standards color and sharpness, even up to ISO 1600, with the advantage of plenty of resolution.
1DX has less noise, but the color and clarity are NOT the same standard of good old 1DS Mark iii.