There are many strong female characters in onepiece including one being a yonko. It is without a doubt that yonko are one of the strongest characters that has been introduced in the onepiece storyline, including a female emperor "BigMom". I believe that there is no point in arguing that Bigmom might be the strongest female character to be introduced to us, apart from bigmom there are many female characters that have a very important and strong role in either past or future of onepiece storyline. Among many characters that have been introduced to us these are some of the characters that I believe are very strong and hence, I have categorized them on the basis of their power level.
15. Koala
Koala is revolutionary army member. She was introduced to us in the Dressrossa arc along with Sabo. Only a hint to the her past and her fighting style has been shown to us. It has already been shown in the flashback that she boarded Fishman Tiger's ship and that is where she learned Fishman Karate. It has only been shown that she uses fishman karate and it has still not been disclosed if she has any devil fruit ability or not. She seems to be a very smart and intelligent character.
14. Charlotte Brulee
Charlotte Brulee is the eighth daughter of the Charlotte Family. She is an interesting character that has been shown frequently in the Wholecake island arc. She has played important role in many scenarios including scene with Luffy and Dogtooth's fight. Brulee has interesting devil fruit power that allows her to enter and come to the real world whenever she likes. She can also take number of people inside her mirror world. She has "Mira-Mira No Mi" that is "Mirror-Mirror No Mi". Her Power makes her extremely powerful inside her mirror world. Even though she was easily defeated in wholecake arc her Devil ftuit power is very hard and annoying to handle. Hence, She is in the 14th spot on the list.
13. Vinsmoke Reiju
Vismoke Reiju is the eldest child and the only daughter of the Vinsmoke Family. She is also known as "Poison Pink". She is the Princess of the Germa Kingdom. She is a mercenary just like her other brothers from the family. Reiju has not been shown much in action but i believe she is powerful as well.
12. Tashigi
Tashigi is one among the least known female swordsman in onepiece storyline. She is a marine officer that works under Vice Admiral Smoker. Tashigi is also captain of G-5 and seesm to be an important character in onepiece. It seems that she doesn't have any devil fruit powers but we came to know that she has gotten stronger after timeskip and she even managed to learn Haki. She used Haki against Monet as well.
11. Hina
Hina is a marine Vice Admiral. Hina is another character that has not been shown much in Onepiece. The last time she was shown was in the Marineford Arc. She is also referred to as "Black Cage Hina" as her devil fruit power allows her to produce black cages. As she has the rank of Vice Admiral it is safe to assume that she has both observation and armament haki like all other vice admirals. Haki is the only reason why I believe she is powerful.
10. Nico Robin
Nico Robin as we all know is a member of Strawhat Pirates. She is an important character in the onepiece storyline as she is the only survivor of Ohara incedent and she is the only person able to read the poneglyphs. She is the Archeologist of strawhat pirates. She has a bounty of 120 million belli and is a devil fruit user that has Hana-Hana No Mi. She has not shown us her full power yet just like Zoro after the timeskip. She was powerful before timeskip as well but she seems to much more powerful as she was training under revolutionary army.
9. Nami
Nami is another Strawhat Pirate who is the navigator of the pirate crew. She was not much powerful before timeskip but she seems to be pretty powerful now as she has been shown to fight against cracker and even Zeus. She was able to bring Zeus under control as well. She is not a devil fruit user like many other in the list but uses a special method to control weather around her.
8. Monet
Monet was the assistant for Caesar Clown and was the member of Donquixote Pirates. She was a Lugia Devil fruit user that had Yuki-Yuki No Mi. She was easily able to overpower Nami and Tashigi as we saw in the punk hazard arc. Due to this I believe she was a poweful female character. The only reason Monet lost was because she faced Zoro. Monet was also able to hold off luffy for long time.
7. Jewelry Bonney
Jewelry Bonney is the captain of "Bonney Pirates" and one of the members of the worst generation.Jewelry Bonney is a suspicious character in Onepiece who has appeared multiple time and has shown her devil fruit ability as well. Bonney has a Devil Fruit ability that can alter her age and thus change an appearance at will. She was also shown to have altered the age of marines officers while she was trying to get off Sabaody Archipelago. All the supernova's seem to very powerful as we have already been shown the powers of other supernovas in multiples instances. Even though nothing much about her past and details of her power have been shown I believe that she is powerful.
6. Carrot
Carrot is the mink character that was introduced to us in the Zou arc. After her Sulong Transformation in Wholecake Arc it can be said without a doubt that she is extremely powerful. As i have mentioned in my previous theory i said that i believe she has not yet fully mastered Sulong transformation and has not fully shown us her power. I believe that was just a hint that she showed us. She was also able to scare both Jinbe and Bigmom pirates hence making her a fearsome character.
5. Boa Hancock
Boa Hancock is a fan favorite character that has a status of Warlord in Onepiece. She is also known as the Pirate Empress. Hancock is the only female Warlord in onepiece storyline. She is captain of Kuja pirates and Empress of Amazon Lily. She is devil fruit user as well that has Mero-Mero No Mi.She seems to be a very powerful pirate as well.
4. Tsuru
Tsuru also known as "General Staff Officer" is a Navy Vice Admiral. Even though she has been shown many times in both the manga and anime her power still remains a mystery. She is respected by all the navy officers and is a very powerful character as well. The only reason I believe she is powerful is that even Doflamingo was scared of her when they were shown in Corazon and law's flashback and she even managed to scare Doflamingo to withdraw back. She seems to be the female version of Garp. Nothing much about her powers have been revealed but she is respected and people are scared of her just like in the case of Garp. She was also shown to be with garp and sengoku in their childhood picture.
3. Charlotte Smoothie
Charlotte Smoothie is the 14th daughter of Charlotte Family and one of the Sweet Commander of BigMom Pirates. She also serves as Totto Land's Minister of Juice. She has a Bounty of over 932 million Belli almost 1 Billion Bounty making her a very fearsome opponent. As we already know that there are not many people having bounty above 900 million belli in onepiece storyline yet. So, It can easily be assumed that she is one of the most powerful female character.
2. Caterina Devon
Caterina Devon also known as the "Crescent Moon Hunter" was the most dangerous female pirate ever to be imprisoned in Impel Down. She is currently a member of Blackbeard Pirates. Her Power, Bounty and Past all are unknown to us but even Fleet Admiral Sengoku was shocked when he knew that Caterina Devon along with other blackbeard pirates escaped Impel Down. She seems to be extremely powerful as welll as she was imprisioned in 6th level of Impel Down.
1. Charlotte Linlin (Big Mom)
Charlotte Linlin also known as "BigMom" is one of the Four Emperors that rule the Grand Line. She is the Captain and Head of Charlotte Family. She is also a devil fruit user that has Soru-Soru No Mi. Even Though her Bounty is still unknown it can easily be assumed that it is far more than Charlotte Dogtooth as well. Since she is one of the four emperor ruling over grandline she is easily the most powerful female pirate.