Why was Han dynasty not included in poll? It was certainly stronger than Tang and Song.
In my opinion, it was the Ming. Why? Because only during the Ming dynasty did China have to capabilities to project power across the world. Even today, the PLAN does not have that degree of power projection. The Ming, if they had wanted to, could've landed navies in Europe and conquered all of Europe. Think of how history would be different then. The Ming were a superpower in every sense of that word. They led the world economically, militarily, scientifically and culturally. They had superior technology to any other people on Earth.
If I had a second choice, it would be the Qing. Under the Qing, the Chinese empire became larger than it ever was. In terms of landmass, the Qing was the greatest dynasty. They controlled all of the modern-day PRC, Taiwan, Mongolia, Outter Manchuria, Korea, Vietnam, Okinawa, and stretches of land in Central Asia and the Russian Far East. For the first hundred years, the Qing could also be classified a superpower, before the European nations surpassed it.
While the Ming was a oceanic power, the Qing was a continental one.