You want to sum up the largest empire, indeed the largest ever sovereign polity, in one word? Contrary to what both neocolonialists and anticolonialists want to believe, the Empire's legacy is mixed.
On the upside the Empire planted the seeds for liberal parliamentary democracy all over the world. It spread major advances in technology across six continents and greatly helped explore the seventh. It filled out a lot of the blank spaces on the map in the second great age of discovery after the initial Spanish-Portuguese efforts several centuries prior. It spread the superior common law system in lieu of continental Europe's more authoritarian civil law approach. We created nations from scratch like Hong Kong and Singapore, Canada, Australia and the United States, that have all become powerful forces, in general, for good in the modern world (I believe, anyway). The combined might of the Empire helped win two world wars and bound the Commonwealth (most of it anyway, silly old Ireland
) together in unity after the breakup of the Empire. Unlike every other European power we generally had a pretty peaceful and amicable withdrawal, excluding several incidents in the run up to Indian independence and the Mau Mau rebellion, but these are pretty minor in contrast to Portugal fighting a long and bloody war in Africa, an almost-coup d'etat in France over Algeria, and Belgium bombing the crap out of the Congo. The Empire's culture fostered fantastic advances in philosophy and society as a response to the industrial revolution, and we treasure much of the musical, visual and literary art of the Victorian era. There is a lot to be said for the Empire.
On the downside? A lot of warmongering, severe classism, and a racist and paternalistic attitudes towards other cultures. We precipitated the oppression of indigenous groups in North America, Oceania and Africa that leaves a stinging legacy to this day. Our desire to expand and conquer drove us to create terrifying military machines like the machine gun, the bomber and chemical weapons, which would claim the lives of millions in two world wars. We carved up Africa after we left with no concern over tribal or religious differences, and these have taken decades to settle - for example Southern Sudan voted just months ago to secede from Sudan after a pointless conflict that could have been easily avoided by drawing up more suitable borders based on the Islam-Christianity divide in Sudan.
Ultimately I don't regret that we forged the Empire, that we spread great ideas, established good infrastructure, a common and beautiful language, and forged bonds transcending continental differences - our relationships with Australia and Canada for example are pretty damn strong for the most part for example, despite being thousands of kilometres away, because of our shared imperial and cultural history. I do wish that we hadn't left in such a hurry without sorting things out properly - the old colonial borders are clearly inadequate for achieving lasting peace in Africa especially, and I wish that we had adopted more of the conciliatory approach that appeared hurriedly in the last days of the Empire. Perhaps we could have been a democratic trans-national union with a population of billions and territories right the way around the globe. T'was not to be