The correct sentence is this: TWO party systems are most common in the English speaking countries of the world.
Two party system refers to a type of party system in which only two political parties dominate the government. At any point in time, one of the political party usually have the highest number of members in the government and such a party is refers to as the governing party while the other party is called opposition. A good example of a two party system is the Republican and the Democratic political parties of United States of America.