The best known monarchy is no doubt the various Commonwealth realms ruled over by Queen Elizabeth. However, she is generally seen as mostly a figurehead, with elected officials being the ones that do the actual ruling.
I'm interested in monarchies where a king or queen has real power. That would include any absolute monarchy, but I'm also including a constitutional monarchy if the restrictions set upon the monarch by such a constitution are limited and the monarch still has a wide amount of power and freedom to exercise said power. If the monarch doesn't have at least as much power as the president has in the USA then I wouldn't consider them a 'real' monarch, at least for the sake of this question.
I realize 'biggest' is a rather imprecise term as well, but I want to leave that open to interpretation. You can use physical size, population, GDP, per capital GDP etc, all I'm interested in is seeing how well some of the more successful modern-day monarchies are doing compared to more modern forms of government.