One Piece has been one pf the best ongoing Anime series for over a decade now and with the crew ever growing in strength as well as number the question one tends to ask is who is the strongest member of the group? With A variety of top contenders on to choose from it is simply narrowed down to three. Monkey D. Luffy (Captain), Roronoa Zoro and "Black leg" Sanji. Out of these three I believe the least likely choice (Sanji) has been foreshadowed and is actually far stronger than even his Captain.
How he fights:
Of all the Straw hat Pirates Sanji is the only member to not have Devil fruit powers or use weapons. a tactical genius when need be He goes as far as refusing to use his hands as it may damage the and prevent him from cooking at a top standard. He has been shown to posses immense physical strength despite being quite thin. He is always confident in his abilities even though he has never had the edge over an opponent due to these restrictions he has placed on himself. His sheer kicking power is so strong he was able to severely damage the invisible man Absolom with a single kick in thriller bark. He was able to do this even though this particular opponent was a devil fruit user and had his body altered by Dr Hogback making himself extremely durable. Finally feats like this occurred consistently for this particular character all pre- timeskip meaning even then he has grown exponentially stronger like his Nakama. Which Leads to the Next reason....
How Sanji's training was different to the rest of the straw hats.
Out of all the Straw hats the two characters who experienced the worst when it comes to training during the timeskip would be Sanji and Usopp. Usopp was trapped on an Island where almost anything can kill you, Sanji on the other hand was left stuck on and island where at times it seemed anything could keep you alive until you decide to kill yourself.... Particularly in Sanji's case as he is quite perverted. For Usopp his problems eventually dulled allowing him to gain what I would consider the best improvement of the crew.As for the others They were all placed in Environment where they would be safe and could constantly train with Zoro and Luffy getting help from the two most perfect candidates to be their teachers (rayleigh and Hawkeyes Mihawk). Sanji was left in constant torment on the island of drag queens with no mentor tailored to specifically to his fighting style he essentially had to learn himself in order to survive what he (and most teenage guys) would deem to be HELL!! Even with all these issues surrounding the circumstances of his training compared to the others Sanji still gained some of the most useful and rarest abilities, Like improving Daible Jambe and gaining the ever awesome Skywalk ability. With Diable Jambe in mind I shall move to my next reason...
Sanji was able to use and control his Haki before the timeskip!!!
Please make note what without the powers of the Gomu Gomu no mi chaces are luuffy would have struggled alot more than he has on his journey thus far. it wasn't until right before the timeskip that he was able to demonstrate Haki and still it was not a controlled burst rather more on the side of pure rage/luck. As for Sanji and Zoro they both provided example of using Haki other then observation before hand with a moderate control over it. Between the two Sanji Presented n has controls his haki more and for longer than Zoro as it is represented in the form of his flaming legs.
Haki is within all beings souls in the one piece world and as passionate as Sanji is about his future, dreams and love(women/Nakama) it is not a shock that Oda represented his Haki with Diable Jambe as it represents passion at it's strongest (love is the strongest force on earth). Since the timeskip Sanji has not required spinning to activate this power and has also gone as far as to cover his whole body in this awesome form of Haki just like Vergo is capable of during the Punk Hazard saga. It has been stated by Oda himself that Sanji's fiery edge is based on his soul as it is ablaze instead of his body (also why his leg does not get burnt. Although Sanji has non honed his full body haki quite yet I beleive it will when he revelas himself as his true symbol.
Currently three straw hats represent god/deities.
Since the star of the series it has been stated that each member of the crew symbolize an animal, but as time has gone by i believe that Luffy Zoro and Sanji have prgressed to represent Gods.
Of these "Gods" Zorro represents Asura -a member of a class of divine beings in the Vedic period, which in Indian mythology tend to be evil and in Zoroastrianism are benevolent.
While it is highly possible but not confirmed that Luffy represents Sun Wukong- The Monkey King, Luffy is destined to be the king of the pirates and his full name is MONKEY D. Luffy!!
As for Sanji he has stated himself that his fire represent the devil who is undeniably the most powerful and most world recognised of the three. In some cases Sporting goat legs a kick from Sanji COUGH COUGH... I mean Satan with legs like that would be torture in itself. Another Example of this is going back to hi training as he called it hell. Or Satan's Domain. He has even had moment where he has looked like the devil and Will most likely be able to do so on a long term basis as his own version of an awakening. If this is the case then this is an obvious foreshadowing of his character backing him up with such powerful connections to pop culture and leaving him as the third strongest member does not make sense.
In the End I think Oda has huge Plans for for one of the most popular one piece characters and I believe one day he could go as far as to match and even surpass the main antagonist of the series if he chose to.