Well... You just answered youre own question lol! It all depends on the prediciment you are in... If you are in a long, drawn out war and Communism is available, you would most likely switch to that. If you are peaceful and building, you would choose either Democracy or Republic. Despotism is obviously pretty bad so you dont want to stay in that very long, and Monarchy is good for waging early wars. They all have their advantages and disadvantages. Personally, I prefer Deomcracy... But sometimes, if im playing on a huge map, with a large sprawling empire, i will use Communism just for the heck of it.