What does O'Brien say the authorities are most interested in within 1984? How does this compare with other leaders (e.g - Hitler, etc..)
Like were the other leader's the same as the authorities and O'Brien or were they harsher?
I am going to refer to the second part of your question.
To achieve this power, O'Brien focuses on two types of torture, a mental torture and a physical torture. The mental concerns itself with lies and anticipation. The physical combines with the mental in that O'Brien chose the rat cage helmet for Winston. This was one of Winston's greatest fears to be around rats. O'Brien was going to let the rats eat his face off. The anticipation of this really messed with Winston, and the pysical experience would have been extreme pain, not to mention that rats carry innumberable diseases that could spread in that type of torture.
I find this type of treatment maybe harsher than Hitler's starvation and labor camp type of treatment, although there may be many things Hitler did that I am still not aware of. If you look at what Stalin did, he just purged or killed many people. I think I would rather die quickly as opposed to being studied to find my weakest points mentally and emotionally and then tortured.
Power can be achieved many ways, but the most effective is through fear. This is not right, it is not good, but it is what has most quickly moved peoples in many of our history's events. I think Orwell uses this book to teach us a lesson about the detrimental uses so we work to achieve through other means that are productive and good for all people.
O'Brien says that the other leaders like Hitler and Stalin were not like the leaders of the Party. He says that those leaders thought that power was a means to an end. They thought that they should get power in order to do something (like to spread communism).
By contrast, O'Brien says, the leaders of the Party know that power is an end in itself. He says that the only reason for getting power is to have power. There is no other goal.
O'Brien says that the Nazis and communists were just about as harsh as the Party, but that they were not brave enough to admit what their goals were.