USA, Japan, and Portugal are quite difficult, IMO. I tend to play with the hightest difficulty but Global Economic Volatility minimized since that seems to cause a much lower starting GNP, for some reason.
The US has some rather serious issues, low GNP, a huge deficit and an economy that needs fixing. Unfortunately, one of the best ways to increase GNP, Stem Cell Research, will anger the mighty religious voters, who might also require serious spending on Married Tax Allowance in order to stay happy.
Japan has fewer issues but instead an astronomical debt which requires some good timing and a bit of luck to conquer without annoying too many voters. Fortunately there aren’t that many pesky religious voters to interfere with my plans. 8)
Portugal on the other hand has a completely run-down economy which needs serious fixing to generate enough taxes to keep voters happy. Here you must find a delicate balance between boosting GNP, pleasing voters, and keeping spending down.
EU, Sweden, and most countries with 5-year terms are easier in comparison.