Great question.
I think you could write a graduate theses based on it (and no doubt many have been). My opinion is that you can’t exclude either the individuals or the times in which they appear.
Lets look at Hitler. Hitler was in many ways a symptom of intrawar Germany, a man of his times so to speak. Most of his ideas were not original, and many of them would have been tried by the first right wing group to come into power. His anti-semitism was certainly not unique (unfortunately), nor was his desire for revenge over WWI. Once the '30s came with all the associated economic turmoil WWII was possibly unavoidable. The majority of the Germans including the armed forces wanted the war. The generals had specifically maintained an army that would allow for a rapid expansion when the time came. In fact many points in the treaty of Versailles were being bent if not broken by the '20s. One example of this is the fact that although Germany was denied an airforce, they maintained highly regemented flying clubs. These were turned into a flying corps as soon as Hitler authorized it. In addition while Germany was not allowed to build warplanes, the producers made models for civilian use which could easily turned into bombers etc. Yes Hitler was the spark, but a spark of some sort was likely.
Now having said all of that, Hitler certainly uniquely affected the war itself. First when war finally did break out Germany was probably as well prepared as anyone could have made it. He had gained control over Austria and the modern Czech republic and their resources. He had provided his troops with practical experience in Spain. And he had left his opponents politically divided. During the war he made decisions (such as the invasion of the USSR) that virtually no one else would have, which drastically affected the war and the post war make up of Europe. So yes there is at least one person who made a difference.
So who else qualifies?
In the political arena I can think of a large number of individuals that drastically changed the course of history.
Alexander- His personal quest for empire, Helenized the Eastern Mediterainian and Middle East. It is unlikely that any other individual would have attempted such a feat. This had significant ripple effects of tying the Leavant and Egypt to the Greek world and eventually leading to their incorporation into the Roman empire. It affected the development of the young Jewish religion and led to the introduction of the Jewish and other eastern religions into the central and western Mediterainian Basin.
Hannibal- After the defeat of his father, Hannibal (and the other Barcas) maintained a grudge against the Romans. This led directly to the Second Punic War. Without this war it is unlikely that the Romains would have gained their empire. All the signs were pointing to a multi-state enviroment similar to modern Europe, with a series of states keeping any one state from being dominant. Instead Rome was able to beat Macedonia and Carthage while keeping the various minor states like Syracuse placated. After this there was no sufficiently powerful and well enough positioned enemy to threaten Rome for half a millenium. Of course other individuals such as Varro and Scopio (father and son) are significant, but without Hannibal a blow out war like what happened seems far less likely.
There are lots of others as well; Napolean, Washington, Khan, Stalin have all greatly influenced the Times they lived in. Political power seems to allow dominant individuals to have great effects in multiple areas.
Outside of politics it seems harder to me to identify individuals or events that hold the same effect. Artists are generally part of movements. Scientists tend to work with others and from previous theories. The laws of motion and thermodynamics are likely to be discoved in the same time frames regardless of the loss of an individual. The silmultanious development of Calculus by two seperate individuals is just one example.
The only other class of individual I can think of with a similar influence is religious. Certainly Paul and Mohammed, as have been mentioned, were unique in their contibutions to hummanity and changed the course of history.
