Rather than looking at the ten most important people I'd look at the ten most important events: And then try to find which person was most important to each event.
That way the process is simplified somewhat (although it does rule out artists, writers etc).
Here's my views, but note that I'm not putting these in any particular order.
A) World War I
Reshaped the european political landscape and redefined how wars were fought.
B) World War II
Reshaped the entire world's political landscape, and changed the course of the rest of the century.
C) Cuban Missile Crisis
The closest moment to armaggedon during the cold war. Showed the strengths and weaknesses of both sides, and their leaders.
D) Fall of the berlin wall
A symbol of the end of communism in Europe and Russia.
E) October revolution
The beginning of the Soviet Union, one of the dominant powers of the century.
F) First powered flight
Began a complete revolution in both transportation and warfare.
G) First mission to the moon
An example of the technological advances achieved during the 20th century.
H) Detonation of the first atom bomb
The signal of yet another kind of warfare, and of a new way of generating power.
I) Civil rights marches
Announced major changes in the democratic process, giving women and various minorities equal rights.
J) Decline of the British Empire.
Changed the lives of billions of people in Africa, India, Australia and other countries.
Now, for the people associated with each of these:
A) Archduke Franz Ferdinand.
His death sparked the war.
B) Adolf Hitler.
Yes, he was only one of many who were involved, but he was the most prominent, and the actions of him and his colleagues led to the war.
C) Oleg Penkovsky.
Provided crucial information to the west, at the cost of his life.
D) Christian Fuhrer.
Wasn't quite sure who to select here (there's so many), but I selected him on account that he was one fo the main organisers of the 1989 demonstrations that eventually led to the opening of border crossings and destruction of the wall.
E) V. Lenin.
Led the revolution in Petrograd and began a series of changes to his country, turning it into one of the wrold's most powerful.
F) The wright brothers.
Successfullly constructed and flew the first powered aircraft, and continued flying it for some years.
G) John F Kennedy.
While he may not have actually piloted the craft, or helped in it's construction what he did do was provide the political vision to begin the Apollo programme. I think it's safe to say that without him ther'ed have been no moon landing in 1969.
H) J. Robert Oppenheimer
Scientific director of the atom bomb project.
I) Martin Luther King.
Stood up in the face of substantial opposition and was a voice to those without rights.
J) Harold McMillan.
His winds of change speech announced a significant change in the way the British governed their empire, and paved the way for independance to be granted for large parts of the world.
