I'm an anthropology major with focuses in history and sociology, so prepare for a long winded speech.
First of all, the word history is defined as thus:
his⋅to⋅ry
  /ˈhɪstəri, ˈhɪstri/ Show Spelled Pronunciation [his-tuh-ree, his-tree] Show IPA
?noun, plural -ries.
1. the branch of knowledge dealing with past events.
2. a continuous, systematic narrative of past events as relating to a particular people, country, period, person, etc., usually written as a chronological account; chronicle: a history of France; a medical history of the patient.
3. the aggregate of past events.
4. the record of past events and times, esp. in connection with the human race.
5. a past notable for its important, unusual, or interesting events: a ship with a history.
6. acts, ideas, or events that will or can shape the course of the future; immediate but significant happenings: Firsthand observers of our space program see history in the making.
7. a systematic account of any set of natural phenomena without particular reference to time: a history of the American eagle.
8. a drama representing historical events: Shakespeare's comedies, histories, and tragedies.
If we analyze the etymology of the word history, it comes up as "his story," as in reference to man's story. On another note, it also denotes a primarily patriarchal society, though that is irrelevant for the moment. This means that history could contain all that has ever had to do with humanity, or it could mean that it deals only with man's past as far back as man has started recording it's doings. However, then we have to take into account the word 'prehistory', which focuses on the time before man' recordings.
So what we have here is a bit of an argument. Naturally, the most important event in history would have to be it's beginnings. So which is it? The beginning of life? The beginning of the universe? Or is it the beginning of man or when he started to record?
If we take my suggestion, then we would go with Genesis (that is, the beginning of the universe). This is primarily due to the fact that everything is connected in a long sequence back to the very beginning, and so is really all just parts of a greater whole object.
