If you thought that exposure was terrible, then what would you say about this image?

The point I am making is that a proper exposure is less about how "nice" an image is when you view it on the preview screen or look at the jpeg on your computer screen or print, and has a mid-gray object in it that looks like it does in reality. But more about recording the important parts of the scene, with detail, in such a way that with some manipulation you can extract all of the details, tones and colors to make a complete image. Mnay photographers are proud to state that they do not spend any time manipulating their images. I say that they are "leaving money on the table" by not taking their images and improving contrast, sharpening, dodging and burning, as the photographer who took the picture below did, in order to create a better image. So the first image is a correct exposure, and the ultimate result bears that out.

My suggested answers to a question with many right answers but even more wrong ones:

As Adams suggested when different technology was involved, the best negative is the one that lets you make the best print.

But we are not using negatives and we are not always printing. Of course, in B&W darkroom work the target paper also varied.

So to extrapolate, the best exposure is that which allows you to make the best print or to make the best digitally displayable final image (usually a JPG, of course). In other words, the best final image quality.

I would not use "middle gray/grey" as my standard for this for at least two reasons.
1. In my inept but empirically oriented testing I found that a middle gray (about 18%) target does not result in a value of 128 (where 256 is max). I have learned through reading that other people's cameras don't necessarily work to the same intended result (some assume that the target is 12%, some 18% and so on).
2. For a variety of reasons, the best image quality will usually be obtained at the "right end" of the possible range of exposures, short of "blown". In this concept, image quality includes "noise" as a negative characteristic. An obvious exception might be when stopping down the lens even one stop would sharpen the picture to a greater extent than the consequent loss of quality due to noise.

Of course, my comments assume that the original file is "raw" and with no more compression than the camera insists upon. Just as obviously, then, my comments assume that the photographer will do post exposure processing. I'm also ignoring color balance issues which can affect exposure in ways that are too complex for me to even understand let alone include here. But my bottom line is that middle gray is not even well defined and does not take into account the dynamic range issue. Adams knew that he had gobs of latitude (AKA DR) and also usually had subjects with a large range of reflected light values. Unless our subjects are similar, taking advantage of the modern digital camera's abilities makes a lot of sense.

As a tactic, using a gray card to set exposure, when you know where in the range of possible "densities" it will fall for your camera makes sense. So does "sunny 16" which is just as scientific as the way that Adams set the exposure for the example given earlier in this thread. But I would not say that it's sufficient, only a good starting point.

Anyhow, there's only the intended result and the technique you use to get there. If they are a perfect match then that is the best exposure.

Generally, a correct exposure captures editable information in the highlight area and in the shadow area of the image.

Overexposure fails to capture enough information in the highlight area for manipulation.

Underexposure as a rule will fail to capture enough information to produce detail in the shadow area.

One source defines correct exposure thus: "The act of having correct exposure means your combination of settings between aperture, shutter speed and ISO speed have produced a perfectly exposed image. When nothing is blown out (highlights) or lost in shadow in an image, it has achieved correct exposure." (found at: https://www.canon.com.au/explore/glossary/correct-exposure)

Hope this reply helps.