I came across this list of the 10 greatest Empires today at http://listverse.com/2010/06/22/top-10-greatest-empires-in-history/

10 Ottoman
9 Umayyad Caliphate
8 Persian
7 Byzantine
6 Han Dynasty
5 British
4 Holy Roman
3 Russian
2 Mogol
1 Roman

If, as the author says, the entries are listed by influence and size then surely the British Empire would come in at first place. What do you think?

I think the Spanish seem to be unaccountably absent, for starters.

I think it's hard to assess the impact of empires, because one always seems to be built upon the ruins of others.