Since we have eminent scholars of international relations who adhere to and promote each of these theories, there is no objective way to answer this question. I would encourage you to think about this on your own to determine which theory you think best fits the reality in the world today.
I would argue for constructivism. This theory holds that international relations are heavily influenced by the ways in which states perceive themselves and other nations. I believe that this is a more realistic (for want of a better term) approach to international relations than realism or liberalism.
Realists hold that countries will always work to increase their power. I simply do not think this is true. The US, for one, often does things that seem unlikely to make it more powerful. It does so because, for example, it sees itself as a democratic nation that should spread its system to the rest of the world. Realists also hold that states act towards each other based on what the other can do, not what it is likely to do. If this were true, the US should be more concerned about the power of England than of China. Yet this is not the case because we see England as a friend.
In both cases, the US seems to act in a way that is determined by its view of itself and its mission in the world as well as by its view of other countries and how compatible those countries are with its own values. This is most similar to what constructivists predict and I, therefore, believe most in constructivism.