When asking 'which is the greatest empire', what do you define as 'greatest'?

[Doctaum]

Hi Everyone,
I'm doing a project with the title 'looking back at different empires how can we asses how great an empire is?' or 'by what criteria can we asses the 'greatness of an empire?'', so it a kind of 'what do you mean by greatest ?' question. I've come here for some help.Any help is welcome, but this is not a 'which empire do you think is the greatest'. I'm looking at things like the culture, legacy, size, time it lasted, economy,population and other points, but please use empires as examples.I'm not much of a historian so please include as much detail as possible, so for example if you say something about culture, say why it is important/what it shows.Thank you very much for any advice or info, it will be much appreciated.

[Valmir]

If you want more Culture and Civilisation take Roman or Greek empires:), And if you want territory get the Mongol Empire.