In relative terms, and biologically speaking; the answer would be stable, complex agrarian societies - the Roman Empire, Medieval Europe, the cultural peak of Islam, China or Japan in the centuries before the modern era etc.
Because in these societies - or so it seems - the intellectual classes usually have the highest reproductive success, which is probably due to lower child mortality, which suggests that they are doing better than almost any other class.
Consequently, average intelligence seems to rise (relative to baseline) in these complex agrarian societies.
But since the industrial revolution, the intellectual classes reproductive success plummeted, at first relatively (compared with the lower classes) then absolutely as fertility dipped further and further below replacement levels.
Although intellectuals have high levels of health, life expectancy, prosperity, comfort and so on - the fact that their reproduction has collapsed, strongly implies that intellectual are maladapted to industrial (and 'post-industrial') societies - since reproductive suppression is evidence (in general) of severe stress, a seriously hostile environment.
Yet, of course, it was intellectuals that created the industrial revolution.
So, intellectuals made a new kind of society which is biologically-lethal to intellectuals.
And as the effects of collapsed reproduction continue to work through ('dysgenesis') then the industrial revolution will stop then reverse; and (probably) revert to the kind of complex agrarian society which is - again - beneficial to the reproductive success of intellectuals.
What was the Achilles heel of the intellectuals in industrial societies?
Atheism - since the only known antidote to reproductive suppression among intellectuals in post-industrial revolution societies is devout traditional religion.
(That is, even in modern societies, traditionally religious intellectuals will - on average - have fertility above replacement levels.)
And atheism is the root of Leftism; and Leftism destroys traditional religion; and Leftism also destroys modern industrial societies.
But whose fault was atheism/ Leftism?
Why - the fault of the intellectuals, of course!
Bad choices- choices of bad; multiplied by hundreds, thousands, millions.