Camera does not take photos. Even the very best (bestest of the bestest) camera can not tell you what to shoot, where to stand, what kind of image you want to get and when to press the button. Technically ANY modern DSLR is capable of the best image quality photograph. ANY MODERN DSLR. The difference in image quality between a 700 USD and 7000 USD DSLR camera is very small. In the real world it is often negligible. You will need to understand, that it is you who take photos, not the camera. This is like asking "What is the very very best hammer to hit the nail on its head."
You need to seriously think about it.
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Mamiya Leaf Credo 80MP Digital Back Kit with 645DF DSLR and 80mm f/2.8 LS D Lens 020-00302A
False. There is a HUGE difference between the ability of a D5100 and a D4s. Huge difference. Just like there's a big difference between a finishing hammer and a ball-peen and a framing hammer. There are TOOLS for the job, and some are better than others.
Probably we differ in our understanding of "HUGE".
My point is, one has to get to a certain level of skill and vision before the difference between "tools" , as you say, becomes important. At this stage a photographer does not ask these questions on a forum, he knows what "tool' he/she needs.
I am convinced that if someone asks what camera they need to upgrade to, 9 times out of 10 they do not need an upgrade. At the very least a guy who needs an advanced camera will ask advanced (specific) questions. When someone asks which camera is the best in the world, chances are (very good chances) he needs a D3300 or a D5200 just because of its tilting screen nothing else. These cameras will be the best in the world for him/her.
For a beginner who does not even know what-ball peen is (btw what the hell is it?), it is all the same: he is going to hammer simple nails into a straight plank in a random order, just for his own pleasure. Yeah, I need to capture my doggy in a jump, give me D4 ! I would say, give me a break. My golden rule - think of an image before you think of a camera. Not the other way round. When you know what image you want to take, you know what camera, sorry "tool" you need to use.
That simple. I, for one never asked what camera should I buy, I know what I need for my photography, nobody knows it better than myself. If I was thinking "well, what camera do I need?", - my biggest worry would be - is there something wrong with my photography? Sadly, a lot of people just want to own a hammer. They like hammers. Once they got it , then they start looking for a nail.
And, by the way, give a D5100 to GaryA or Derrel or some other guys here, will all know who I mean, they of course will look at it thinking "what the hell is this", but then will produce a perfect image of a doggy in a jump. Simply because they know a thing or two beyond AF speed numbers.
End of rant.