Dictatorship until my civ gets big, then communism. Damn, where did that copy go?
Rebublics and democracies do moderately well given most circumstances. They usually end up with the will of the people, which is usually what counts. A down-side is the red-tape it produces.
A dictatorship or monarchy (very close) can do extremely well or poorly, depending on the person in charge. What is needed is a strong personality that works towards the good of the people. A dictatorship always has a strong personality, though usually working for it's own ends. A monarchy tends to work towards its own ends, though not always, and usually has a series of terrible monarchs punctuated by brilliant ones. (Read Machiavelli's "The Prince", it tells you all sorts of useful things. He also wrote somethign from the standpoint of a republic, the name name eludes me)