Boa Hancock is the only female member of the now-defunct Seven Warlords of the Sea, and consequently is among the most powerful female pirates in One Piece. She rules the island of Amazon Lily, home to the Kuja, a tribe of warrior women, where men are forbidden to enter.
Hancock's power is disguised by her often comedic role in the series, including her falling head over heels in love with the naive Luffy. In reality, her abilities are first-rate, thanks in part to her Devil Fruit and a mastery of Haki.
Hancock's Devil Fruit
Hancock was force-fed the Mero Mero no Mi (Love-Love Fruit) while enslaved by the World Nobles. It's a Paramecia Devil Fruit which allows her to turn anyone attracted to her into stone. Hancock's beauty is legendary, and thus the fruit's powers work on nearly everyone she encounters, regardless of gender. This can be done in a variety of ways. At long range, Hancock can fire a heart-shaped beam (Mero Mero Mellow) or heart-shaped arrows (Slave Arrow) to turn opponents to stone, or by blowing kisses at them. She can also turn enemies into stone at close range by striking them physically.
Hancock can also restore anyone she petrifies to normal. The Mero Mero no Mi does have a few limits on its power. People like Luffy are immune due to having a pure heart, and other extremely strong pirates like Sir Crocodile and Dracule Mihawk seem able to resist its effects. Vice Admiral Momonga was also able to counteract its effects by stabbing himself in the hand, with the pain he felt overriding any feelings of lust. While Hancock's Devil Fruit is powerful, the way she uses it in conjunction is what makes her especially dangerous.
Hancock's Abilities
On top of her Devil Fruit, Hancock is an extremely proficient hand to hand fighter, who like Sanji seems to prefer fighting with kicks. Her strikes are powerful enough to break stone and easily dispatch the extremely durable Pacifistas. Like other Kuja warriors, she is also a master of long-range combat, which makes the aim of her Slave Arrow attack nearly perfect.
Hancock is also a proficient user of Haki, and is one of the few people in the world who can use all three types. These include Observation Haki, which allows her to sense things other's can't and read opponent's moves, Armament Haki, which she uses to enhance her physical attack, and the extremely rare Conqueror's Haki, an ability she shares so far only with Luffy, a small group of other extremely powerful pirates, and former Fleet Admiral Sengoku.
As far as the Warlords go in terms of strength, Hancock likely ranks near the top, not quite approaching Yonko-level strength like Mihawk. Still, the fact that she defeated several Pacifistas with ease puts her solidly above weaker warlords like Buggy the Clown and Gecko Moria. Hancock also has the benefit of having her own island base (and is the only Warlord currently in possession of one) and a large standing army to go along with it. Her sisters Marigold and Sandersonia are also masters of Observation Haki and are Zoan Devil Fruit users, adding to the strength of her crew significantly.
Hancock was so assured of her and her crews' strength that she chose to fight when her status as a Warlord was revoked and the Marines attempted to seize Amazon Lily and arrest her. It's unknown yet how this battle turned out, and what Hancock's current fate is. Some inferences can be made, however. Hancock's Devil Fruit effectively renders any numbers advantage over her moot, with only those able to resist her charm posing a serious threat to her. It's quite possible that she managed to protect Amazon Lily or escape from it unscathed. Only time will tell for sure.