For me, it would be "King" (actually the Queen...) Hatchepsut of Ancient Egypt.
When her husband the king died, the heir to the throne was her nephew; and he was only a boy at the time. She could have allowed the priests and other advisers run the kingdom until young Thutmose could do the job himself, but she didn't...she did the job herself.
She ran the kingdom, set up new trade and tribute agreements with neighboring kingdoms, built temples, monuments and other public buildings, and repaired old tombs and temples that were falling apart.
At the time, some of the things she wanted to do could only be ordered by the "king." So she dressed in the King's crown, false beard, and ceremonial clothes...effectively making herself the king in order to get things done that needed to be done.
Egypt prospered under her rule, but it was all for naught....
After her death, her successor (remember her nephew Thutmose?) erased her name from all the temples and public buildings, defaced or destroyed statues representing her, and had her name removed from the official "king list."
I'd like to think she wouldn't have cared about that...that she did what she had to for the sake of the kingdom...but I guess we really can't know that.