I am shocked, utterly, that no one on this thread has advocated for the British Empire as the most powerful empire in world history.
By power: I mean only "dominance," Dominance is not merely a matter of military might in-and-of itself, but the manner of its execution. Dominance is likewise, not merely a military matter, but ideological, cultural, and economic. In this sense, war is waged as much with money and religious conversion as it is with the sword.
The British Empire had the largest empire in terms of % of land under its control at its height of any other empire that has been listed on this thread (23.84% of all land). It had nigh universal and uncontested rule over the entirety of the oceans for the majority of its existence unlike any of the other empires on this thread with the possibly exception of, and for a much briefer period, Spain. The British easily make the top ten as far as % of population ruled (23%), but only this low because we trust projections as to the world's populations at the the time of, lets say, the Archaemenids which are said to have ruled 43% of the world's population (but how the fuck could they possibly prove that?).
Given the speculation involved in population numbers at the times of the ancients and where these ancient empires were (given we do not have census information for, lets say, ALL of Europe at this time) it seems hard to assume these numbers as correct and that the British are only #10 on population ruled. This being the case, the British Empire may even be higher on % of population ruled and could be even in the top 5, and among periods that we have accurate census data for, should possibly be considered even #1.
Further, even if conceding that they only made #10 on population ruled, in controlling the #1 largest portion of land and sea, by far, in human history, the British demonstrate themselves, by force of reason, to be the most powerful in virtue of the fact that controlling a large expanse of land is more difficult than necessarily controlling a large population. Indeed, Many weak and impoverished nations of small stature have controlled large and dense populations, but only powerful military empires have ever ruled large tracts of land, the largest being that of the British.
Likewise, The British Empire's influence on economics in establishing capitalism as a world-system and Protestantism as a global phenomena in Africa and Asia cannot be discounted either and may be the greatest testament to their true dominance. Not to mention, that the most widely used lingua-franca in the history of lingua-francas is undoubtedly English, the most predominant shared cultural media is also, ultimately, English-derived (even if mediated through Americanism), and the current global empire of the United States is a natural inheritor of the British culture and is more of a continuation of that system than the Byzantines were of the Romans.
Most of the other praised European empires of the last 500 years, such as France and Spain, were defeated and replaced overtime by the British. This empire, was the largest empire and maintained such through two world wars and reached it height in both land mass and population during the bloodiest, most increasing, and most advancing period in human history (the early twentieth century), even after establishing its Imperial dominance, to some extent, in a period as primitive and as far removed from our present time as the era Elizabeth I. Not to mention, that no Empire ever lost as much of its Empire only to come back to its greatest height as the British did in losing the Americas only to then rule most of the world and only within roughly 100 years. This is unprecedented.
I'm sorry gentlemen, but there is no question.
The British Empire was the most powerful Empire ever, and The Anglo-Sphere, broadly-speaking, remains the most powerful civilization currently and possibly in the history of mankind.