Relativism
But the question asked the most "benevolently righteous regime possible", not the most benevolent regime period. The most benevolently righteous regime possible (MBRRP) would necessarily be a compromise between differing notions about its nature, I think.
The "most benevolent regime period" is practically synonymous with "the most benevolent regime possible" - the key is in the "most" part, implying that we can only get so far in either case.
Regardless, you would still have to define not only benevolence and righteousness by subjective criteria but also how you're going to measure it. Even saying "the regime that most people think is most benevolent" still isn't objectively the best criteria to judge it by. Certainly that minority who didn't agree about the level of benevolence would complain about using a majority vote as a measure. Take the UK election system. The Liberal Democrats want to change to a proportional system because if you define it like this it's the best. Tories want to keep it as is, because if you define it like that it's the best. Even merely saying "compromise" can be tricky, because you run the risk of diluting the ideas down so far that they can't be properly executed and are ineffective. Such a compromise idea between all opinions would lead a system that pleased no one because everything that an individual wanted would be stopped short by another persons wants and needs. We see such examples all the time where proposed laws are compromised on and weakened, and we end up with final drafts that no one particularly wants. The law doesn't go far enough for its supporters and goes too far for its opponents - despite shining public faces of "well, we made a good compromise so it's all great" everyone, deep down, is actually unhappy about it.
I you imagine "most" as the maximum on a curve, then it's easy to find where you want to be for any particular system, regime or otherwise. However, because of choosing your definitions by subjective criteria, and these change between people, there isn't just one curve but several, and their maxima are found in different locations. A compromise situation would end up with a curve with no maxima, but a mere straight and flat line, probably very low down.
And that's without getting into specifics about defining a benevolent act. In one particular model, if you want to define "most benevolent" as just reducing harmful acts to an absolute minimum, then the "most benevolent regime" would, in fact, be no regime at all - as something that can't exist cannot be malevolent in any way.
We let the super-intelligent "Minds" decide how to execute policy, with the possible exception of going to war. As to what policy should be, yes, there will always be disagreement, but ideally the mantra would be "if you think you'd be better off in a different system, don't interfere with those who don't, or go elsewhere."
Both of these would only be optimal or even possible in a society that lacked serious cultural/moral heterogeneity and had done away with money, poverty, illness... I think perhaps you're thinking more about the here and now, when I'm thinking more abstractly about the (distant) future.
(I think my point is far more abstract that you seem to think it is!)
Which God decided that "intelligence" (regardless of how high) is a required criteria for deciding benevolence? Would they have a better idea about how someone would want to be treated than that person themselves, for instance. Again, it's all basing it on ultimately arbitrary criteria.
I would think it would be more like this: each individual who opts to live in this "benevolent regime" would essentially decide however to live their own life, under the guiding principle that "my right to swing my fist ends where the other man's face begins." Ultimately you can't make everyone happy, so those who simply cannot abide this regime would be forced to leave it.
These criteria are arbitrary. It's arbitrarily an anarchist-communist utopia based on hedonism. However, I believe that under that arbitrary system the most benevolently righteous regime would be able to function, i.e., that is the best form of society.
I can't justify this point of view scientifically or logically, because doing so would require too many a priori assumptions. Why is intelligence necessary to make good policy? I suppose you could get lucky, but that's not a government I would put much faith in.
This is a world much like an projected Lumenosian afterlife, called the Meif. Meif means max initial equal freedom. "People" interact in worlds wherein the physics are decided by contract beforehand. One of the possible problems is the inability to redeem the lost soul. The bad people try to entice people into contracts wherein they end up enslaving themselves. Some may end up in one of these hells by their own decision.
Oop, my bad. I meant the most benevolently righteous regime that actually exists in the world today. I shall update it. I always seem to screw things up this way.