Who? What? When? Where? Single most important event in your life including any or a few of all four.
Out of all of the above, the major event I will list first with a few others as well:
1. Beng Saved and Baptized
2. Miracle of Birth (when told by a Chief of Obstetrics in a very well known hospital that I would never have any children.
3. The initial stage of marriage when vows actually meant something. (Not everyone experiences this)