Tropico 4! My favorite city building Sim since SimCity 2000. I loved the original Tropico, but Tropico 4 is such an improvement on just about every level. I'm enjoying the Campaign quite a bit, but Sandbox is fun too.
For those who don't know, in Tropico, you are your very own island "Presidente". You can choose your background and how you rose to power, as well as certain traits, and then decide how you're going to rule your island.
For example, you could have been the Generalissimo of the previous administration, giving you soldier efficiency and the respect of the Militarists. Perhaps you came to power by overthrowing the previous military regime in a bloody military coup. You might add the "Veteran", "War Hero", and "Vigilante" traits to maximize the respect of the military. You then might immediately impose Martial Law, suspend elections, and initiate a bloody reign of terror, executing those who dare to protest against your rule.
Or you might be a Oil Tycoon who won a capitalist election, exploiting the island's natural oil deposits to turn a tidy profit.
I've played with great success both as a communist exporting raw metals and cash crops and as a capitalist, refining those cash crops into rum (from sugar), cigars (from tobacco), and/or canned coffee (from coffee, duh), and refining those metals into jewelry (from gold) or weapons (from iron). I've played brutal dictators and charismatic democracy-loving presidents, and the middle ground between the two.
It's just a lot of fun.