The story of One Piece revolves around the Straw Hat Pirates. Sanji is one of the main characters in the series. He joined the crew during the Baratie arc and became the cook of the crew. Sanji is an excellent fighter in addition to his cooking skills.
Also, Sanji is a smart character. He is the most clutch Straw Hat after Luffy. There have been several instances where Sanji got the crew out of a pinch. But, there are instances where Sanji has let himself down as well as his fans.
10 SMART: SAVED CREW FROM BANANAWANI
When Crocodile had imprisoned the Straw Hats and Vivi inside his base. The crew was destined to die within an hour. It is fair to say that most of the crew would be wiped out if Sanji hadn't intervened. Sanji called Crocodile and introduced himself as Mr. Prince. With a little help from Chopper, Sanji was able to rescue the rest of the crew without fail.
9 NOT SMART: LEFT THE CREW WITHOUT TELLING THEM THE TRUTH
When the Straw Hats separated at Dressrosa, Sanji, and co. went towards Zou where they encountered some members of the Animal Kingdom Pirates. Sanji and co. made short work of them and the Minks were indebted to them. However, things took a turn for the worse as Capone Bege arrived at Zou. He gave Sanji with an ultimatum and Sanji had no other choice but to go with him, however, Sanji did not reveal the full details to Nami and others. He just told them he would be back, but if Luffy hadn't chased him to Whole Cake Island, he would have been killed.
8 SMART: WENT AFTER NICO ROBIN
Nico Robin's history was a big problem for her. She was chased by the World Government ever since she was a kid. At Water 7, she was pursued by CP-9, who took her to Enies Lobby.
While the rest crew was in disarray, Sanji went after Nico Robin. He found out that Robin was on the sea train. He told Nami about the situation via a transponder snail. Sanji boarded the sea train and beat several World Government agents including Wanze.
7 NOT SMART: WENT TO SEE MERMAIDS
At Fish-Man Island, Sanji's life was in peril. After seeing real women for the first time in 2 years, Sanji couldn't help but appreciate their beauty. This put Sanji at great risk as he was getting continuous nosebleeds. Chopper advised Sanji to not see any mermaid or his condition could worsen. Sanji completely ignored his advice and continued to see mermaids. Sanji's condition only got worse and Chopper once again reiterated that Sanji should stay away, but Sanji ignored him again. This is one of the few instances where Sanji's gag was taken a bit far.
6 SMART: BLEW AIR INTO KUROOBI'S GILLS
When the crew arrived at Arlong Park, they had to face the powerful fishmen. Sanji's opponent during the arc was Kuroobi. Despite Kuroobi's repeated boasting about how fishmen were superior underwater, Sanji jumped into the water to save Luffy. Underwater, Sanji was easy prey and Kuroobi would have killed him if Sanji hadn't conjured up a plan to save himself. Sanji blew air into the gills of Kuroobi which allowed him to break him and head for land. Once Sanji was back on land, he defeated Kuroobi comfortably.
5 NOT SMART: PREFERRED NOT TO FIGHT KALIFA
Sanji morals have sometimes caused him a lot of problems. At Enies Lobby, Sanji faced Kalifa before fighting Jabra afterward.
What should have been an easy fight became one of the most humiliating defeats for Sanji just because Sanji doesn't hit women. Sanji was washed away because of Kalifa's newly acquired powers.
4 SMART: OPENED GATES OF JUSTICE
One of the toughest tests that the Straw Hats faced was at Enies Lobby. They had to fight against CP-9 and several thousand Marines. The momentum shifted several times during the fight. Usopp was saved only because of Sanji's intervention. After Luffy defeated Rob Lucci, the crew had to leave Enies Lobby, and they wouldn't have managed it if Sanji hadn't found the way to open the Gates of Justice.
3 NOT SMART: FED GIN
Sanji's principles caused him problems again when Gin came to Baratie. Instead of throwing him away, Sanji gave him food. It was a humane decision, but not a smart one. Sanji's decision would come to haunt him and Baratie as Gin returned with his captain, Don Kreig, and the rest of their crew.
2 SMART: MESSED UP ARK MAXIM
Enel would have wiped out everyone at Skypeia if not for Luffy's intervention. Enel's devil fruit was naturally weak to Luffy's Gomu Gomu no Mi. Another thorn in Enel's side was Sanji, but not because of his fighting skills. Sanji's extraordinary ability to plan things in advance once again bailed out the Straw Hats. He saved Usopp and Nami from certain death. Also, he managed to sabotage the controls of Ark Maxim, which caused Enel to lose his cool.
1 NOT SMART: LEFT WITH VIOLET
Violet met Sanji during the Dressrosa arc. Sanji was taken by surprise when Violet kissed him. Sanji was easily swayed by Violet's actions and even with his perverted character, it was surprising. Sanji was walked right into a trap, which he could have very easily avoided.