"What do you reckon are the best man made structures?"
It depends on what you mean by 'best' and 'structures'. When I say 'best' I could mean historically significant, largest, most complex or perhaps most beautiful, and this list is hardly exhaustive. By 'structure' I think, Copago you were implying a static building but ships, aircraft and even sculpture are structures too? So I guess I get to choose the question I am going to answer.
The most complex structures which human beings have designed and built are; the Space shuttle, aircraft carriers and the giant offshore oil & gas platforms. Aircraft, rockets and ocean liners all being hugely complex but somewhat simpler structures. Of these despite being so much less than its original conception, so frail and so dangerous I would nominate the space shuttle.
The most historically significant structure may well be the Great Wall of China. However, I think instead I would like to suggest the triplet of the Channel Tunnel (again something where reality is far less than conception), the Great Belt Bridge and the Oresund Bridge. In the era where the dream of European unity was strong they brought the continent of Europe closer together. I think the dream is probably dead now but it was a beautiful one.
The largest is easy it has to be the US freeway system.
The most beautiful structure is very difficult to choose. I think it may well depend upon where you live for many people. The Opera House, The Eiffel Tower, the Statue of Liberty and of course the Blackpool Tower are all symbols of the places in which they stand and the people who built them. However, there are two structures that bring out the greatest emotional response in me personally and I guess that is the best measure of beauty I know. Firstly, in Oslo the Vigeland sculpture garden. It is a huge site filled with hundreds of statues all the vision of a single artist taking human life as his subject. Secondly, and more significantly crewed space rockets. To watch these huge towers of fuel and oxidant rise toward the heavens on a lance of flame is almost a humanist benediction. From the grace and elegance of the early Soviet Vostocks to the controlled savagery of the once mighty Saturn 5s they are all beautiful.
NB: Bertho, I remember asking you this once before why are you mystified over the construction of the pyramids?