From a personal point of view, its season one, where she had her hair long. Infact many of my online friends in a Long hair forum, are still trying to work out how they did her hair in those first few seasons. (NB the bun on top is called a Gibson)
From a military point of view, a lot of the hair cuts just would not be put up with. She would have had to wear her hair above, and not touching the collar. Season 1 and the first few others got it near to being right, neat, tidy, off the collar up dos, however later on the hair was allowed to grow on and beyond the collar, this would be totally unacceptable. It is one of the only disappointments I have with the series.
PS Was it a wig or extensions attached to her own hair?