For me I have two contenders.
First is Margaret Beaufort, at just thirteen years old she was a widow with child, a child who would later go on to be Henry VII, the first Tudor king, bringing an end to 300 years of Plantagenet rule. She had to negotiate her way through the War of the Roses, through the competing claims of Henry VI, Edward IV and Richard III. Frankly I don't know how she stayed alive, especially after a failed attempt to overthrow Richard III before the later successful attempt. She was also able to negotiate the marriage of Henry VII to the daughter of Edward IV thus reuniting the roses. She went through various political marriages in order to achieve this and then actually outlived Henry VII to see the coronation of her grandson Henry VIII.
Kick ass.
The second is Ching-Shih, who worked as a prostitute in a floating brothel in Guangdong before marrying Cheng I, a pirate of the South China Sea. Together they formed a huge coalition of competing pirates into one Red Flag Fleet to dominate the area. They adopted a son, Cheung Po Tsai - Chow Yun Fat plays a representation of him in Pirates of the Caribbean - who she also married on the death of Cheng I essentially to ensure she continued to run the show. She lay down strict codes including no theft from villages, no rape and equitable sharing of stolen goods. She defeated the Qing Navy, the British Navy and the Portugese. She was finally defeated by the Portuguese and the Qing dynasty said she'd be granted amnesty if she just quit pirating about - and she was like 'sure', and settled down in Macau and opened a gambling house.
Kick ass.
Who would be your pick and why?
