Based on the reading, which "name" or period of his life had the most significant impact on history?

The most significant period of Malcolm's life, according to his autobiography, is when he turned from being a "Little" to adopting the surname "X." This symbolized his liberation from white society.
After his conversion to Islam and release from prison, Malcolm became a prominent figure in the Nation of Islam, a black separatist organization. He is most famously associated with this movement, though he later broke from it. During the era of the civil rights movement, he spoke out strongly in favor of blacks, using violence to protect and advance their rights.
This put him at odds with Martin Luther King, who advocated a purely non-violent approach to improving race relationships. Malcolm thought that didn't make sense and said things such as:

Concerning nonviolence, it is criminal to teach a man not to defend himself when he is the constant victim of brutal attacks.

While Malcolm was greatly moved and changed in the direction of reconciliation between the races after his pilgrimage to Mecca near the end of his life, he is most known for the more militant time before that experience.