The Classical Greek philosopher Plato (428 - 348 BC) wrote about five forms of government, which progressively may degenerate starting with aristocracy and ending in tyranny. Please feel free to associate modern governments with any of Plato's forms.
Plato favored form of government was an aristocracy, a state ruled by an enlightened leader imbued with the highest ideals and grounded on wisdom and reason. Many current long-time heads of government may see themselves as such enlightened souls. According to Plato, an aristocracy may degenerate into a timocracy, a state ruled by those primarily motivated by personal honor or advantage. Its more extreme form is the plutocracy in which power is used to entrench wealth. From there it's not far to an oligarchy, a two-class society of the rich as the governing and the poor as the governed. An oligarchy will eventually degenerate into a democracy in which the weight from the majority of the lower classes transforms the society towards the indulgence of personal freedoms and the consumption of whatever one desires. We finally then end up in a tyranny where there's no discipline and chaos reins supreme.