Bratticuss hit the nail on the head. Despotism and Democracy are all you ever need - all other forms are utterly useless.
If you go for early conquest then you stay at Depotism, fill the land with closely packed cities and crank out chariots (+ some diplomats and sails) until you own the world.
For anything else, you beeline to Democracy and stay there for good. See that you are at war or w/o contact with all opponents before you make the switch. Also, you make sure to get Religion with Mike's chapel and Bach ASAP.
The rationale is very simple: Democracy gives not only the highest bonus of all government forms, it is also the only form which is both decentralized (and thus oblivious to distance of capital - like Communism) and allows for city growth via those all-important WeLoveThePresident-days (like Republic). The limitations of Democracy are easily overcome with proper planning, a good (rail)road network and frequent use of the h-key. If you build Shakespeare in a production rich costal city to support your ships and expedition forces, you can field huge armies with no penalty whatsoever. - Not that you actually need one: conquest with Democracy means taking out the capital and buying up the rest, so your invasion force will consist mostly of diplomats and settlers.