He brought them wealth, power and reproductive opportunities the like of which they'd never seen before.

"reproductive opportunities"...brilliant, very well-phrased...
He was also a horribly cruel and murderous man who spread misery wherever he went, so subjectively we're not going to want to credit him with success.
Now, why on Earth should misery be subjective?
No just kidding. On the other hand (misery being objective or subjective), I suppose sitting through
The Coronation of Poppea (four hours long) is an awesomeand awe-inspiring smorgasbord of sights and sounds for a highbrow opera lover; for those who are not, the Eighth Amendment was just what the doctor ordered.
That said, I think we can objectify this, but to do so we'll have to throw away a few misconceptions, like our "western" tendency (or maybe American tendency?) to think of compassionate leaders as the only possible "good" leader. In fact, Machiavelli faced this by using the word "virtu" I may have mentioned. It might have really meant "virtue" even though the much-underlined and dog eared copy I have translates it as "ability" (but the footnote explains the use of the actual word and why that translator thought that should be the translation into English rather than its cognate, "virtue").
But I think an able leader is a virtuous or "good" one. Americans are often accused or flattered (or both simultaneously) of being "idealists" rather than "realists". By the way is anyone on Redscape from Italy? Just out of curiosity?
Personally guys, I think you can objectify a good leader, to a great extent. But I cannot ignore the fact that yes, to pass judgment on a leader does indeed require a degree of subjective thinking. Also, once we identify what is subjective it makes it a little easier to come to terms with what a good leader ought to be, even if we have to take a certain amount of context, or take that sort of "virtue" with a grain of salt. That grain of salt can be where and when.
Conquest is not a trait of a good leader, or a bad one, necessarily. For example, I did a lot of reading on the Byzantine Empire, back when I could read whole, massive books (I got halfway through Lord Norwich's 1,200 page epic on the subject, from the division of the empire by Diocletian, to the conquest by the Turks in 1453; and I even took a class called Byzantine Civilization). They seem to have had good and bad emperors.
I was considering making a short list of the more notable ones, but that would get too far off topic. And not all of you might have the same background of info on the Byzantines. I studied them more than their predecessor emperors (the Roman Roman Emperors).
I'd write more right now, get a little more in depth, but it's 4:48 a.m. and I'm having trouble focusing via sleep deprivation.
But closing thought, I agree on the subjectivity, but as long as the subjectivity is limited to within time and place, I think that one can proceed to objectify.