The Straw Hat Pirates are one of the strongest crews in OnePiece. After journeying to multiple islands, the crew has grown in strength. The strongest three members of the crew are: Luffy, Zoro, and Sanji. Together they are known as the Monster Trio. Sanji joined the crew towards the Baratie Arc, and he's been an ever-present figure ever since. Sanji was recruited to be the cook of the crew, however, his fighting style is just as spicy as his food.
Like every member of the Straw Hats, Sanji continues to develop with every passing arc, and he will undoubtedly play a big role in Luffy's journey of becoming the Pirate King. Towards the end of the series, Sanji is going to fight a strong opponent, but so far there has been no hint towards who could Sanji possibly fight towards the end.
Sanji's Difficult Past
The recent arcs in One Piece have been kind to Sanji as he was once again involved in the action. The Whole Cake Island arc in particular revealed many things about Sanji's past. It was important for Sanji's development as a character, and it cleared up a lot of important misunderstandings surrounding Sanji's character.
Unlike the other Germa kids, Sanji was deemed a failure as he was able to show his emotions without any problems. Sanji was kind to everyone including small animals such as rats. Vinsmoke Judge considered Sanji a failure and imprisoned him so that the rest of the world would not know about his failure of a son. Sanji was rescued by Reiju, who broke him free and helped him to board a ship. This ship was later attacked by Zeff and his pirate crew, and it changed Sanji's whole life. Under Zeff, Sanji became a chivalrous man, who could cook and fight.
Sanji's Strength So Far
The Whole Cake Island arc focused mainly on Sanji's character development instead of his strength, albeit he did receive a raid suit. It wasn't until the Straw Hats reached Wano that Sanji began to demonstrate his power. Sanji fought against Page-One, who is a member of the Tobi Roppo, and managed to overwhelm him rather easily. It is important to note that Sanji wasn't accustomed to his Raid Suit fully at the time of their fight.
After the Straw Hats launched an attack on Onigashima with their allies, it became clear that Sanji was going to fight against the big names this time. Sanji came up against Queen, a member of the All-Stars. What made the fight even more interesting was the fact that Queen worked with Judge in the past and he knew about the experiments. During their fight, Queen used all the abilities that Sanji's brothers possess, however, Sanji still did not use his Raid Suit. So, in a way, Sanji was trying to prove that he was more than just an experiment, and he did not need his father's science. Sanji eventually managed to defeat Queen and gained his first big victory since entering the New World.
Future Strength And Opponent
After his fight with Queen, Sanji is only going to improve further. Among the Straw Hats, Sanji is the second-best user of Observation Haki. He has demonstrated that he can use Observation Haki expertly as he managed to save Tashigi, avoided Katakuri's jelly bean, and managed to sense Nekomamushi's aura.
Sanji will definitely look to improve it and take it to Katakuri's level, where he can see the future. This would be the next step in his development as a fighter. Sanji will also need to improve his Armament Haki, as he is not as good at using it. After destroying the Raid Suit, and awakening his genetic powers, it will be interesting to see what else Sanji is capable of doing in the future.
So far, there has been no hint regarding who could be Sanji's final opponent. Many fans believe that Sanji's final opponent could be Admiral Kizaru, as the two of them have been seen battling each other in one of the OPs. There are others who believe it could be Jesus Burgess, but it is important to remember that it is Zoro, who fights the right-hand man of the enemy. So this leaves Shiryu, who is the captain of the second ship of the Blackbeard Pirates. While Shiryu does use a sword to fight, it is important to remember that he ate the Suke Suke no Mi, which allows him to turn invisible.
When it was revealed that Absalom ate the Suke Suke no Mi, Sanji was furious. It is fair to say that he would have the same reaction when he found out about Shiryu. Also, Sanji's high level of Observation Haki would make him a better match for Shiryu. Furthermore, with the help of his exoskeleton, Sanji can tank a few hits from Shiryu without getting into serious trouble. It is fair to say Sanji's fight against Shiryu would be an outstanding one, and it will cement his status as one of the greatest fighters in the series.