great doesn't have to mean this person was a nice guy. One can do terrible things and still be "great". Some of the people that come to mind are: Julius Ceasar Hannibal Napoleon Genghis Khan Atilla Alexander David Plenty of others, tell us who you think and why.
it is, but its hard to imagine an answer for the question. Those Persian dynasties where some of the greatest.
Constantine. Restored and reformed the Roman Empire and ensured its continuation for hundreds of years following his death.