By almost any measure, Saudi Arabia takes the prize.
Most of the absolute monarchies or even monarchies where the monarchy has more than symbolic power outside a rare constitutional crisis are in the Middle East and North Africa (Brunei and a few African micro-states would be the other main exceptions). Specifically:
the world’s current absolute monarchies are Brunei, Eswatini [f.k.a. Swaziland], Oman,
Saudi Arabia, Vatican City, and the seven territories of the United
While not strictly an absolute monarchy, Morocco has a king with real power, as do Jordan, Kuwait, and Bahrain.
Saudi Arabia is a larger country by any measure, than any of the others.
The Roman Catholic church holds sway over more than 1 billion people, but not in the sense that the question implies. It is an absolute monarchy without secular competition over only a few thousand priests and lay religious officials in tiny Vatican City, and its absolute monarchy's succession is not hereditary.
North Korea is arguably one of the world's largest absolute monarchies but is not formally organized in that manner, even though its rulership has passed from father to son twice and the ruler of North Korea has practical power comparable to an absolute monarch.
Even then, however, Saudi Arabia probably edges out North Korean in terms of country size by most measures.
Saudi Arabia has 34.8 million people, 800,000 square miles, and a GDP of $1.6 trillion ($46,762 per capita) and many international allies. North Korea has 25.8 million people, 46,540 square miles, and a GDP that is not well estimated, but is almost surely less per capita and overall than Saudi Arabia. North Korea is highly isolated internationally.
Both North Korea and Saudi Arabia have moderately powerful military forces. North Korea has nuclear missiles (experimental), a large navy, and a high level of mobilization, but low-technology, out-of-date equipment, and poor training for its troops. Saudi Arabia has a smaller military without nuclear weapons, but what it has is state of the art.
I'm interested in is seeing how well some of the more successful
modern-day monarchies are doing compared to more modern forms of
It is worth noting that the most recent wave of revolutions against non-democratic regimes in the Middle East and North Africa, sometimes called Arab Spring, was more successful in the case of non-monarchies (e.g. Libya and Syria) than in the case of monarchies in the region.