Well, this is a very tough question to answer seriously.
In terms of the brutality and destruction that an empire can cause at war, I think the overall victor there has to be the Mongols. No other empire that I can call to mind has in history, been as ruthless or as vicious as them.
In terms of efficiency I would have to say that the Romans were very impressive on that front. Army's that were so well trained a mere handfull could swing the tide of battle just using superiour fighting methods and tactics. Also advancements developed by the Romans are rather astounding, many of which are still used today, such as the advancements of roads, aqueducts, bronze tubing for pumps, cement that could be used underwater, reinforced concrete and massive advancements in the ideals of Law and Order. These are major advancements that we still use today and are by no means anywhere near the full list. A more exhaustive list can be found here (http://www.mariamilani.com/ancient_rome/ancient_roman_inventions.htm#List%20of%20Ancient%20Roman%20Inventions)
And finaly with reguards to size, the undisputed choice has to be the British empire. No other Empire in history has been as large or as expansive as this, a feat that's not easily achieved... as can be seen by the downfall of the empire due to exhaustive war.
Overall these 3 would come at the top of my list for the above stated reasons... but if I were to pick one to top the list, I think it would have to be the Roman Empire. Although not the largest empire it was still vast, and although not as vicious as the Mongol hordes, it was certainly much more efficient in combat. On top of that the above mentioned inventions sway me to choose them.
P.S. These are purely my opinions and current knowledged beliefs, I did no research at all for writing this so if anything is indeed incorrect feel free to correct me.
In terms of the brutality and destruction that an empire can cause at war, I think the overall victor there has to be the Mongols. No other empire that I can call to mind has in history, been as ruthless or as vicious as them.
In terms of efficiency I would have to say that the Romans were very impressive on that front. Army's that were so well trained a mere handfull could swing the tide of battle just using superiour fighting methods and tactics. Also advancements developed by the Romans are rather astounding, many of which are still used today, such as the advancements of roads, aqueducts, bronze tubing for pumps, cement that could be used underwater, reinforced concrete and massive advancements in the ideals of Law and Order. These are major advancements that we still use today and are by no means anywhere near the full list. A more exhaustive list can be found here (http://www.mariamilani.com/ancient_rome/ancient_roman_inventions.htm#List%20of%20Ancient%20Roman%20Inventions)
And finaly with reguards to size, the undisputed choice has to be the British empire. No other Empire in history has been as large or as expansive as this, a feat that's not easily achieved... as can be seen by the downfall of the empire due to exhaustive war.
Overall these 3 would come at the top of my list for the above stated reasons... but if I were to pick one to top the list, I think it would have to be the Roman Empire. Although not the largest empire it was still vast, and although not as vicious as the Mongol hordes, it was certainly much more efficient in combat. On top of that the above mentioned inventions sway me to choose them.
P.S. These are purely my opinions and current knowledged beliefs, I did no research at all for writing this so if anything is indeed incorrect feel free to correct me.