The Roman Empire scores as my favourite.
Here are a few historical generalisations which relate to just how advanced the Romans were.
-It was not until the 15th century that we surpassed the Romans in scientific knowledge
-It was not until the 18th century that Europe surpassed the Romans in terms of infastructure
-It was not until the 19th century that sewers were re-introduced
-It was not until the 1950's that Europe surpassed the Romans in cultural diversity
Western Europe is still yet to experience 300 years of peace.
Special mention goes towards the British Empire, for being the largest and probably last "great" Empire. It lasted little over a hunderd years, but during that time we shook the world, and the legacy we left behind in terms of international trade, global politics, language, sport, justice, technology, science, fashion and literature, will last a good many many centuries into the future. Historians in the future will not be able to explain the two world wars, the industrial revelution, Middle Eastern and African politics, English as an international language and the rise of America without mentioning the British Empire.
However the British Empire will be rememberd in history will be interesting, after all we were far from a positive moral force in the world, but then again, neither were the Romans, but they are rememberd well.