Not a self made millionaire, but I have seen plenty of people starting with significant advantages to crash and burn. The amount of money that can be spent on dodgy business projects, or, worse, various harmful substances, is insane.
IMHO by far the best "significant advantage" you can have is a shrewd, practical brain combined with low levels of blind arrogance and some kind of "people talent". In normal peacetime conditions, people like that tend to rise fast.
Being born in the late 20th century and in a developed, wealthy country is are both massive advantages, even if your parents are middle-class school teachers (as mine were).
If those two facts constitute "significant advantages", then the obvious answer is "none of them", but that doesn't seem a productive place to draw the line.
I didn't introduce those terms here. I'm speaking of the 'wealthy elite', not some arbitrary dollar figure cut off. The point is that 'person born to wealthy parents continues to be wealthy' is still a common story and 'person born in rags becomes a billionaire' is non-existent. We are fed a narrative about hard work and earning, but the people at the top largely only got there by being born from the right womb.