The Dark-Dark Fruit, also known as the Yami Yami no Mi, is a Logia-type Devil Fruit that grants its user the ability to control and manipulate darkness, as well as the power to nullify the Devil Fruit powers of others. While it is considered to be one of the strongest Logia-type Devil Fruits, it is difficult to definitively say that it is the strongest.

Comparing the Dark-Dark Fruit to Other Logia-type Devil Fruits
Logia-type Devil Fruits are considered to be the most powerful type of Devil Fruits in One Piece due to their ability to transform the user’s body into a specific element or substance and control it at will. The strength of a Devil Fruit ultimately depends on the user’s skill and creativity in using its abilities, as well as the strengths and weaknesses of their opponents.
One of the strongest Logia-type Devil Fruits is the Goro Goro no Mi, which grants the user the ability to control and manipulate lightning. This Devil Fruit is considered to be incredibly powerful due to its speed and range of attack, making it difficult for opponents to defend against. The user of the Goro Goro no Mi, Enel, was able to effortlessly defeat many powerful opponents, including the entire Shandia tribe.
Another powerful Logia-type Devil Fruit is the Magu Magu no Mi, which grants the user the ability to control and manipulate magma. This Devil Fruit is considered to be incredibly destructive, as it can melt through almost anything and create powerful eruptions that can destroy entire islands. The user of the Magu Magu no Mi, Sakazuki (also known as Admiral Akainu), is one of the strongest characters in the One Piece series.
The Mera Mera no Mi is another powerful Logia-type Devil Fruit that grants the user the ability to control and manipulate fire. This Devil Fruit is considered to be one of the strongest due to its destructive power and the versatility of its abilities. The user of the Mera Mera no Mi, Portgas D. Ace, was able to hold his own against powerful opponents such as Admiral Aokiji and Blackbeard.
Strengths and Weaknesses of the Dark-Dark Fruit
The Dark-Dark Fruit is an incredibly versatile and powerful Devil Fruit due to its ability to nullify other Devil Fruit powers and manipulate darkness. The ability to nullify other Devil Fruit powers gives the user a significant advantage in battles against other Devil Fruit users, while the ability to manipulate darkness allows the user to create powerful black holes that can suck in anything in their path.
In addition to its offensive abilities, the Dark-Dark Fruit also grants the user immunity to physical attacks and the ability to absorb other attacks and damage into their darkness, making them almost invincible. However, the user of the Dark-Dark Fruit is vulnerable to attacks that are not based on Devil Fruit powers, such as physical attacks or attacks using weapons.
Conclusion
While the Dark-Dark Fruit is considered to be one of the strongest Logia-type Devil Fruits in the One Piece series, it is difficult to definitively say that it is the strongest. The strength of a Devil Fruit ultimately depends on the user’s skill and creativity in using its abilities, as well as the strengths and weaknesses of their opponents.