The term "dictator" is associated with non-monarchal absolute ruler, especially where the base of power lies with the military or possibly a nationalist ideology. Since women have less military power even in the modern era, it makes sense we really don't see that many people we can call "female dictators". We tend to see two types of women on these lists: elected politicians who declare states of emergency, and the powerful (and hated) wives of dictators who usually don't have any formal power independent of their husbands. It has come up before; see this thread, for example.
Indira Gandhi is often on these lists in the first category. It is worth noting that many liberal constitutions have clauses allowing for a (temporary) suspension of the rule of law in what's called a "state of emergency". The Nazis, for example, took power during a state of emergency. Abraham Lincoln suspended habaes corpus in a state of emergency-like move (though this was later declared unconstitutional). Gandhi was certainly a controversial figure in modern Indian history, and many of her political opponents called her a dictator (which is a common critique of opposition parties in democracies, but in this case had more merit than most). One of the reasons she declared a state of emergency was a corruption scandal that threatened to force her resignation. However, I do not think that Indian courts have found her declaration of the State of the Emergency, or her action during it, unconstitutional (I am no expert on India though). She ended the state of emergency after two years. While she lost the 1977 after ending her emergency powers in an election that was characterized about "dictatorship or democracy", it's worth noting that she was reelected in 1980. With Gandhi in particular, does invoking a (constitutional) state of emergency automatically make one a dictator? If so, then a democratically elected woman Kamla Persad-Bissessar just declared one in Trinidad & Tobago in 2011 for, I think, six months; Gloria Macapagal-Arroyo declared one in the Philippines in 2006 for one week.
The other category you see up here a lot is wives of dictators, who often have their own base of power within the regime and are particularly hated within the regime, but don't usually actually rule as an autocrat. Juan Peron's first wife Eva and his second wife Isabel (Isabel actually ruled for a period, though it's widely alleged that one of her ministers actually ran the country); Jiang Qing, Mao's wife and part of the "Gang of Four"; Elena Ceaușescu, the last head of Communist Romania Nicolae Ceaușescu's much hated wife; Imelda Marcos, the wife of Ferdinand Marcos who was long dictator of the Philippines (she's back now as a democratically elected politician after a period of exile); etc.
Arguments could be made for some of those women being dictators, or for most of them wielding "dictatorial powers", I'm not sure there has ever been an unambiguous female dictator. Unless a better case can be made for Indira Gandhi, I think the best case I think is for Isabela Peron, but it's acknowledged that while she was President (1974-1976), José López Rega set much of the regime's actual policy, and it should be noted that she came to power in legitimate succession after her husband's election. There is not any unambiguous case of a female dictator that I am aware of, and I think the case for calling either Gandhi or Peron a dictator are rather weak.
In addition to /u/yodatsracist, there is a case to be made about the Empress Dowager Cixi who effectively controlled China from 1861-1908 during the Qing Dynasty.
She started as the concubine of the Xianfeng Emperor. Because she could read and write, she was able to help the emperor in his older age. It meant she was a bit more politically savvy and well-read than the other concubines and even the empress. Towards the end of the Second Opium War, the royal entourage fled Beijing, as it was under attack by the British, and the emperor died.
Cixi gave birth to the Xianfeng emperor's only son, and after his death, her son became the emperor (Tongzhi). The Tongzhi Emperor was only 5 at the time, so there was a regency council. However, Cixi aligned herself with the late emperor's wife, and they amassed more power than the council. During the ensuring power struggle, she had three of the eight ministers on the council killed, and the rest dismissed.
During her time in power, she dealt with the Taiping Rebellion, thought to be one of the bloodiest wars in human history, and the Boxer Rebellion, among other smaller rebellions. China fought in the Sino-French War and Sino-Japanese War, both losses.
She outlived her son, and placed her nephew on the throne and continued to rule China. In 1889, she handed over the power to her nephew, Emperor Guangxu. But in 1898, she resumed power and ruled China.
The Guangxu Emperor died the day before Cixi, and she selected his successor, Puyi (the Xuantong Emperor). Puyi would become the last emperor of China, and would rule Manchukuo as a puppet of Japan during World War 2.
She's generally not well-regarded. She was corrupt and power hungry. There's even a possibility that her 1908 death was by assassination instead of a stroke.
Follow up questions: /u/yodatsracist mentions that there are no (unambigulous) female dictatorial powers till date. However, the world has seen many female monarchs in different kingdoms across the world.
Why does this disparity exist? For example, even to this day, the Queen of England comes to mind.
Have any of these empresses (let's call them that for the time being) been founders of their dynasties? That would mean that they had established their rule through military might as opposed to family ties, and would make it the loosest historical description of a 'female dictator'.
Indira Gandhi has already been mentioned.
Apart from her, Imelda Marcos is the most compelling example. Whilst she was never the formal leader, her husband Ferdinand fell ill during 1980 and most consider that Imelda ruled from then until the Marcos' were deposed in 1986.