One Piece has introduced a plethora of Devil Fruits that grant ridiculous powers, but few are as dangerous and niche as Boa Hancock's. The Mero Mero no Mi effectively turns any opponent to stone if even a fleck of desire toward her exists, which makes her essentially untouchable to most enemies. This makes the prospect of a perfect counter interesting.
Surprisingly, the character who could be the final opponent for Hancock is Admiral Fujitora, because he is blind and therefore is immune to Hancock's charms. He also has the ability to manipulate gravity, so he can force Hancock to fight at a long distance and take away her undoing ability of pleasing the eye.
Disclaimer: The article reflects the opinion of the writer and includes spoilers from One Piece.
How Admiral Fujitora could be Boa Hancock's final opponent in One Piece, explained

In One Piece, Boa Hancock is one of the show's strongest women. This is primarily due to her Mero Mero no Mi, which uses desire against people. Her beauty and Devil Fruit ability mean that she can petrify enemies at a glance, leaving her nearly invulnerable in battle. But there's obviously a catch to this power: it only affects people who develop feelings for or lust after her. That's why Admiral Fujitora could be the ideal last opponent for her, and perhaps the most threatening.
Fujitora's blindness utterly negates the visual component of Hancock's abilities. As he cannot see her, he can't be subjected to her charm-based attacks, which are activated visually.

This removes her primary advantage in any fight, leaving her dependent on her physical abilities, Haki, and close-range fighting. Hancock is an excellent fighter, but she has never really had to fight anyone because her Devil Fruit abilities do the part.
Aside from that, Fujitora's gravity powers based on the Zushi Zushi no Mi are deadly, particularly in distant battles. He can attract meteors from the sky or immobilize his foes on the ground with brute force. In a battle against Hancock, he could command the battlefield at a distance, preventing her from closing in and employing the use of her other abilities.
She would have to contend with his aggression without her usual psychological edge, something no foe has ever compelled her to undergo before.

Story-wise, Fujitora is also an interesting pick for Hancock's last opponent. Being a Marine who publicly criticized the World Government's actions against the Warlords, Fujitora is a morally ambiguous character similar to Hancock.
Their fight may not just be about strength, but more importantly, a commentary on justice and authority. As Hancock battles for her people and self-determination, Fujitora battles for a justice system cleansed from corruption.
If One Piece decides to grant Hancock a climactic battle that pushes all aspects of her personality, then Fujitora is the most well-rounded and symbolic foe. He can't be seduced, can't be bullied into submission, and serves as an ideological mirror to her. He's almost a foil to not only her abilities but also to what she stands for in so many ways.
Final thoughts

Boa Hancock's Mero Mero no Mi makes her almost untouchable in One Piece, as it turns anything with a desire for her into stone. However, Admiral Fujitora may be her perfect counter. He is blind, which means he cannot be hypnotized by her charm-style attacks and could use his gravity to engage in a long-range battle with her so that she could not use her advantage as a physical attacker.
A clash between the two could be much more than a physical clash but a symbol of personal liberty versus justice. Fujitora may be the one adversary Hancock cannot overpower.
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