The first applicable Google result, To Rule the Earth... applies the term 'empire' too extensively for my taste.
Then I come to the one I like the best, which is, not surprisingly, the Wikipedia entry List of largest empires.
Landmass isn't very impressive if you mostly rule a bunch of yak-populated steppes. ;-) OTOH, the Mongol's beat pretty much every force they came in contact with, so that raises them on the military power scale.
For population, military power, landmass, and longevity you pretty much have to bow before the British Empire. Of course, they were the major beneficiaries of guns, germs, and steel and the last of that era. Like Chirol said in his comment, you can't enslave millions of natives with a few guns anymore.
As for the greatest in no order, we can safely assume the biggest and best were the Romans, Ottomans, Mongols and British.I'm inclined to favor percentage of world population as one of the most important factors. Based thereon, should the Qing, Song, and Mughal empires be in the running?
So, as I continue to think through my criteria, let me throw up a ranked proposal:
- Percentage of world population
- Military power
- Multicultural domination