I would put in these definitions:
How long did their power reign last and how vast was their empire, in land and in people under the empire.
Then you factor in cultural identity etc.
I mean, Alexander's Empire by all logic was the greatest in human history simply because he conquered all of the known world and promoted interracial marriages.
The Ottoman and Mongols had relatively vast empire, large influence and last for a while.
While era should also be factored in, I would put British up there, but their heyday was roughly a century, if that.
You can include the United States if you like with commercialism it has generated, I mean, you see Pepsi and Coke everywhere nowadays don't you?
But since the Cold War the United States does have a foot in most doors.