When it comes to One Piece characters, their strength is mainly judged on the basis of their Devil Fruit ability and the type of Haki they possess. While Devil Fruits grant unique abilities to the ones who consume them, a character's true power is determined by their prowess as a Haki user.
Several characters in the vast world of One Piece have already reached their limits in terms of strength. Some have just grown old and are not in their prime anymore (regardless of their monstrous vigor even at old age) while others didn't have much potential, to begin with.
8 Bellamy
Bellamy has always been on the sidelines ever since he made an appearance in One Piece during the "Jaya Arc." Before timeskip, he possessed strength that could barely rival that of someone like Sanji. Bellamy getting demolished by Luffy has always been one of the most iconic moments in the anime.
As a user of the Bane Bane no Mi, a Paramecia-type fruit, Bellamy has the ability to transform parts of his body into a spring. Regardless of how convenient and powerful it might sound, he fails to utilize his abilities to the most. Furthermore, his cocky and over-confident nature is what limits his potential. It is very unlikely for him to see a boost in his strength.
7 Zeff
Red-Leg Zeff is the former captain of the Cook Pirates and is known to be one of the toughest pirates of his time. Even as a cook, he resorted to violence in the kitchen or while dealing with his underlings. However, like Sanji, Zeff is infamous for only using his leg(s) to fight.
When the Red-leg and Sanji were about to drown to their deaths during a shipwreck, the former lost one of his legs while trying to save the latter. This event surely nerfed the character's strength considerably, considering that he only used his lower body in battle. Back to the present, in One Piece, Zeff is a retired pirate and the owner of the Baratie. It is very unlikely that he will see a spike in his strength as a golden ager.
6 Emporio Ivankov
Emporio Ivankov, the Queen of the Kamabakka Kingdom, or the land of Queers, has one of the most interesting Devil Fruits of all, which gives him the title of a "Miracle Person." As a user of the Horu Horu no Mi, a Paramecia-type Devil Fruit, Ivankov has the ability to alter a person's hormones. He uses his fingernails as syringes to inject hormones into a subject (or himself) and potentially change their gender, grant them some newfound ability, or even heal them.
However, this does come with a catch. The hormones take a toll on the body, especially the healing hormone, which is extremely painful to bear. It solely depends on the subject's potential and to what extent they can get a boost. This leaves Emporio Ivankov, himself, with almost negligible room to grow in terms of strength.
5 Big Mom (Charlotte Linlin)
Big Mom, one of the former Yonkos, is already a great force to be dealt with in the world created by Eiichiro Oda. Additionally, her strong pirate crew and influence make her one of the strongest antagonists yet.
However, when it comes to her own strength, the captain of the Big Mom Pirates is not at her prime anymore and has been defeated by Law and Kid. With her being stripped of the rank of a Yonko, her influence has gone down as well. As per recent developments, it is unclear whether she is alive.
4 Kaido
As a user of the Uo Uo no Mi, Model: Seiryu, rumored to be the strongest Mythical Zoan-type Devil Fruit, Kaido is already strong enough. Although, that's not his only power. He is also known as the strongest creature alive. Along with his exceptional use of Haoshoku Haki, beastly strength, and Devil Fruit power, he can easily be crowned the strongest foe Luffy has ever faced.
However, with Luffy smashing Kaido into the lava pit and liberating the Wano Country, the former Yonko was stripped of his position along with Big Mom. Both the Emperors fought at their limits and were still taken down. This proves that they are way past their prime and the New Generation has surpassed them.
3 Silvers Rayleigh
The first mate of the former Pirate King, Silvers Rayleigh, is one of the strongest characters in the entire series. Despite the fact that he is old and retired now, he is still considered one of the top-most wanted pirates by the World Government.
The Dark King acted as a mentor to Luffy during the timeskip gap and trained him in the utilization of his Haki. When it comes to his strength and capability in the present day, he can surely be considered a formidable foe but is not unbeatable. Due to his old age and retirement status, there is hardly any room for growth in terms of power. So, it is safe to say that he has plateaued in strength.
2 Sengoku
As a former fleet admiral of the marines, Sengoku is still strong in his old age and possesses the power of the Hito Hito no Mi, Model: Daibutsu. His Mythical Zoan-type Devil Fruit enables him to transform into a Daibutsu (a giant Buddha) and get an enormous boost in strength.
Sengoku is physically strong enough to hold down an infuriated Garp and also take on the Blackbeard Pirates, as seen in the "Marineford Arc." Regardless of all his feats, he has his limits and is very old currently. Thus, it is safe to say that he will not see a boost in his strength.
1 Monkey D. Garp
The father of the infamous leader of the Revolutionary Army and the grandfather of one of the current Yonkos, Monkey D. Garp is among the strongest marines ever. At his prime, he was said to rival the power of Gol D. Roger.
Garp the Fist once joined forces with the Pirate King against the Rocks Pirates, 38 years before the current events, during the God Valley Incident. Regardless of how strong he is, the Hero of the Marines is now old. He has his limits and wouldn't be able to grow stronger than he already is. However, some One Piece fans hope that he gets to serve justice to Ace by beating Akainu at some point.