Are you looking for the most aggressive warmonger or the most able?
In my experience the undisputed number one warmongerer is Montezuma, with Shaka following on place two. Only then do Genghis and Ragnar come into play. Gandhi is the obvious choice as your ally, though Washington should do too (America being arguably a weaker Civ than India).
So if say Montezuma ai fought Shaka ai who would win?