The development and use of the atomic bombs by the United States to end the war against Japan can also be at least partially responsible for spurring competition by the Soviets in that area as well as others. Combined with the American and British failure at Yalta to understand Stalin's expansionist intentions and I think you can say the US and Britain share the blame for the Cold War's onset.
I think #4 provides a balance to the other posts above. Certainly we cannot gleefully shift all blame onto Marxist ideologies. The USA emerged from World War II as a superpower that was eager to maintain its world dominance, and clearly was beginning to challenge Russian supremacy even before the end of this war.
I think the United States has to take some of, if not all the blame in this scenario. Our need to maintain our power and our sense of ethnocentricity after WWII meant we were never to going to leave the Soviet Union alone. In addition is it the rise of nuclear weapons (which WE used in WWII) that led to the more serious nature of the cold war, always threatening to make it a "hot" one.
If you're going to blame one person, blame Karl Marx and/or Lenin. The Marxist-Leninist ideology can be blamed for much of the Cold War.
Marxism-Leninism holds that the whole world will eventually but inevitably become communist. It also holds that it is important for communist countries to hurry the process along by helping other countries become communist. The Soviet Union was built on the foundation of Marxism-Leninism. Because of this, it had the attitude that it should spread communism across the world. If it had been less aggressive in this way, there would have been less reason for the West to fear the Soviets and the Cold War would have been less likely to happen.
It is difficult to say that any one person was responsible. The first person who comes to mind is Joseph Stalin, who commented at the end of World War II that the war against Fascism had ended; the war against Capitalism had begun. Stalin refused to withdraw troops from Eastern Europe and even attempted to starve out West Berlin in an attempt to take all of that city. It was only the Berlin Airlift that stopped it. Stalin's continued threats of aggression, and the Soviet's development of an Atomic Weapon led the United States to counter that development by building more arms itself. The end result was the policy of "brinksmanship," that is, going to the brink of nuclear war. For the first time in our history, the phrase "mutually assured destruction" came into vogue.
Stalin cannot bear the burden alone, however. At the Yalta Conference, Franklin Roosevelt, who was very sick, dying in fact, made a number of concessions to Stalin that he never should have made. He was intent on getting a committment from Stalin to enter the war against Japan, and believed--wrongly it turned out--that he could trust Stalin. Also, some blame must be laid at the feet of General Dwight D. Eisenhower. When the Soviets and Americans were closing in on Berlin, Eisenhower elected not to take Berlin, but let the Soviets take it instead. He said it was not strategically important. Later when he was President, he came to rue that decision.
To apportion blame of who started the Cold War would take a lot of research delving into the British, US and Russia archives and personal papers of major political and military figures. This may provide clues into the differences that emerged amongst the leaders of the UK, USA and USSR during the Yalta and Potsdam conferences.
Further clues might be found when the western powers sent troops to combat the Bolsheviks after World War 1. The West viewed Russia with suspicion even after the Bolsheviks defeated Wrangel's army in 1921.
The US and former USSR used surrogate regimes to further their own ideology and national interests after 1945. These ideological differences were manifested during the Greek civil war Korean and Vietnam Wars with the US supporting the "democratic" forces whereas the communists receiving assistance from the USSR.
The Cuban crisis precipated by the Soviets almost brought the two superpowers to the brink of nuclear warfare where cool heads prevailed at the end.