Importantly of course, it will be the case of how do these images look at 100% (ie, viewing the finest level of detail) but I show the whole image for each below so you can see how they came out. These were shot on a Nikon D800 in RAW with no post processing done. Straight out the camera then.
Detail in the brick work and hands of the clock and the contrast of the images places I think the Sigma 300mm as the best of the three images but it is very close with the Nikon 200-400mm. Given the Nikon was handicapped by 3 stops of poorer light during its test, it is a pretty strong result (albeit from the most expensive of the three lenses).
Aside of proving it, the result should not have been in doubt: a sub £1000 prosumer lens versus a £2k prime and £5k pro zoom, the 28-300mm should produce the weakest image but what really comes through is just how close the 28-300mm is at the long end to two best of class 300mm in the widely more expensive glass. For most people, this will be 'proof positive' that there's little point spending so much more money to get so little extra benefit. If you don't need those extra stops of light, it's a fair argument.
Overall then, testing the 28-300mm at both the 28mm end and the 300mm end shows real quality that is impressive on a lens with a 10x zoom range and that costs what it does.