Sanji has mingled with pirates since his childhood. Among the earliest characters to join Luffy's ragtag group of adventurers, Sanji has been around since nearly the very beginning of One Piece. While his backstory would not be explored properly until the Whole Cake Island Arc, the Straw Hats' resident chef has constantly been one of the best parts of Eiichiro Oda's manga and its anime adaptation.
Sanji is funny, strong, honorable (to a certain extent), and does not hide the fact that he loves women. In fact, he would probably be perfectly fine sailing with Nami and Robin only, although he has grown more accepting of the Straw Hats' male members. If things played out differently and Sanji found himself free to join any crew to ever exist in One Piece, which ones would suit him?
10 Love: Roger Pirates
Sanji's ultimate goal is to discover the All Blue, the part of the ocean where all four seas happen to meet. For a cook who travels the waters, the All Blue is the ideal spot; an area that is home to theoretically all the fish in this universe. Whether the All Blue exists or not has yet to be confirmed, but if there is one crew that might be able to answer that question, it would have to be the Roger Pirates.
As Gol D. Roger's crew sailed the entire Grand Line, it has to be assumed they stumbled across the All Blue.
9 Hate: Arlong Pirates
Any human would hate to be on the same island as the Arlong Pirates, let alone to form part of the crew. Originally members of the Sun Pirates, Arlong's crew had little patience for any species besides fish-men, although they did tend to show some respect to humans who were forced to join their crew.
Arlong was a cruel dictator who would conquer weak villages and require their citizens to pay a high tax. Naturally, Sanji would not be fine with following this captain.
8 Love: Spade Pirates
Prior to becoming one of Whitebeard's Divison Commanders, Ace had his own crew called the Spade Pirates. Although not especially strong, they earned a reputation as a rookie crew with the potential for greatness.
From what has been shown of the Spade Pirates, they seemed to have a similar spirit to the Straw Hats. However, they lacked one important crew member, as the Spade Pirates did not have a cook. If Sanji happened to meet Ace before Luffy, he could have very well ended up joining the Spade Pirates.
7 Hate: Krieg Pirates
As the main antagonists of Sanji's debut arc, the Krieg Pirates were essentially designed to represent everything the cook hates. Despite having plenty of personnel, the Krieg Pirates lacked individual strength or any semblance of harmony.
Don Krieg, the crew's arrogant captain, used intimidation to force smaller East Blue crews to sail under his flag. Krieg is not above killing members of his own crew, as he sees no value in life other than his own.
6 Love: Bonney Pirates
Sanji might have a complicated past, but he is a fairly simple man. Time and time again, Sanji has proven incapable of resisting female characters. That personality trait would be enough to convince a pre-Straw Hat Sanji to join Jewelry Bonney's crew.
The fact the captain loves to eat would also make this crew all that more appealing, presenting Sanji with an obvious way to contribute to the Bonney Pirates. Considering what happens to this crew, it is a good thing Sanji ended up with the Straw Hats.
5 Hate: Hunger Pirates
One Piece: World Seeker introduces the Hunger Pirates, a crew that mainly exists to present Luffy with something to punch during a side-quest. The game does not really explore the Hunger Pirates in any significant way, but they seem to go around town stealing food.
Sanji might sympathize with a crew loaded with hungry pirates, but he has exactly no patience for a group willing to steal from children. Regardless of his age or affiliation, Sanji would dismantle the Hunger Pirates.
4 Love: Cook Pirates
Prior to settling down at Baratie and later leaving with Luffy, Sanji worked on a cruise ship that fell at the hands of the Cook Pirates, led by Zeff. Soon after, a storm eviscerated Zeff's crew, sparing only the captain and Sanji. While the two got off to a rather terrible start, they eventually formed an unbreakable bond over their shared belief in the existence of the All Blue.
At the time, Sanji was still too young to really join a pirate crew; in fact, Zeff played a crucial role in helping the young cook become stronger. In another timeline, Zeff and Sanji might have sailed the Grand Line in search of the All Blue together.
3 Hate: Thriller Bark Pirates
Usually, Sanji keeps a cool head in fights, preferring to avoid making things too personal. However, the cook lost it when he went up against the Thiller Bark Pirates' Absalom. Not only had the zombie eaten the Clear-Clear Fruit, a Devil Fruit that grants invisibility, but Absalom sought to marry Nami.
Putting aside all that, Sanji would gain nothing from joining a crew driven by its captain's desire to grow an army. To fulfill his dream, Sanji needs a crew that is willing to travel the Grand Line.
2 Love: Kuja Pirates
If Boa Hancock offered the current Sanji the opportunity to join the Kuja Pirates, he would consider leaving the Straw Hats to join the all-female crew. Consisting of pirates from Amazon Lily, the Kuja are constantly striving to preserve their autonomy from the World Government. They do not seek fame or power, only survival and freedom.
Sanji's idea of paradise presumably resembles Amazon Lily. While he would love to join the Kuja, Hancock's crew would unquestionably reject the cook. Sanji would also die from all the nosebleeds.
1 Hate: Big Mom Pirates
If Sanji never joined the Straw Hats, he probably would have still crossed paths with Charlotte Linlin's crew. As a Vinsmoke, Sanji would have been annihilated by Big Mom and her pirates during his preplanned wedding to Charlotte Pudding.
Beyond the character's personal history with the crew, Sanji's kind nature conflicts directly with Big Mom's selfishness. More importantly, a cook of his caliber would have no freedom as part of the Big Mom Pirates.
