First you need to identify what Stalin's four methods were: indoctrination and propaganda, police force, censorship, religious persecution. This question is somewhat of an opinion. If you can back up your answer with facts, any of these four could be argued to be the most dangerous. For example, propaganda is dangerous because it helps to shape people's overall opinion positively or negatively regardless of whether the facts are true or false. Indoctrination convinces people that they need to follow blindly to what their leaders are saying or doing regardless of whether it is morally appropriate. Police force can be dangerous because the power can be abused and be used to control people by fear. By persecuting people's religions, you are challenging an important belief and moral system that the people use.
