There isn’t one universal “best” aperture. The idea that lenses are generally sharpest at f/8 is not reliable—many lenses peak around 1–2 stops down from wide open, some at other settings. The best aperture depends on the specific lens and camera, so lens tests or your own comparisons are the right guide.

For pure resolution, use the widest aperture that avoids significant lens aberrations. But in real photography, maximum chart sharpness is only one factor. If your subject has depth, you may need a smaller aperture to get enough depth of field, even if diffraction slightly reduces peak resolution.

So the practical answer is: don’t treat the diffraction-limited aperture as a target. It’s more of a warning that stopping down further may gradually trade resolution for depth of field. Choose aperture based on the balance you need between lens aberrations, diffraction, and depth of field.

Subject distance and focal length matter mainly because they affect depth of field. ISO and exposure matter too, since sharpening or diffraction compensation can increase visible noise. For critical archival work, shoot RAW, test your specific lens/camera combination, and use the widest aperture that gives both acceptable aberration control and the depth of field you need.