The following contains spoilers for Eiichiro Oda's One Piece, available in English through Viz Media and Crunchyroll.
One Piece has been one of Shonen Jump's flagship series ever since the beginning of its serialization in 1997. With over one-thousand chapters, Eiichiro Oda's pirate adventure masterpiece is still going strong twenty-five years later.
After a month-long hiatus earlier in the summer, One Piece has returned, bringing with it the beginning of the final saga of the series. Nearly every arc in One Piece ends with a spectacular battle, usually concluding with Luffy beating the snot out of the arc's big bad. With the end of the series inching closer, fans are constantly speculating about who Luffy's final opponent will be. As the plot threads come together in the climactic final saga, there are a few potential match ups for the captain of the Straw Hat Pirates.
Marshall D. Teach, known as Blackbeard, is perhaps the most theorized possibility for Luffy's final opponent. Blackbeard's murder of fellow crewmate Thatch during his time as a member of the Whitebeard Pirates directly led to the events that resulted in the death of Luffy's brother, Ace. Blackbeard killed Thatch because he was in possession of the Yami-Yami no Mi, a devil fruit that gives its user control over darkness, something Blackbeard had sought since childhood. Ace left the crew to pursue him, resulting in his capture and subsequent death at the hands of the Marines.
Blackbeard plays the long game, biding his time until it's the right moment for him to strike. He's sneaky, but exceptionally powerful. With the recent revelation that Luffy's fruit contains the abilities of a sun god, the symbolism of a battle between light and darkness makes this potential match up very plausible.
While Blackbeard may have caused the events that directly led to Ace's death, it was Admiral Akainu that actually killed him. Ace's execution was the catalyst for the events of the Summit War, where Ace's crew, the Whitebeard Pirates, besieged the Marine headquarters of Marineford to rescue him. During the ensuing battle, Akainu goaded Ace into a one-on-one fight after he besmirched the captain of the Whitebeard Pirates.
Akainu possesses the Magu-Magu no Mi, allowing him to control magma. Though Ace possessed the Meru-Meru no Mi, which allowed him to control fire, the sheer power of Akainu's devil fruit was too much for Ace. When Akainu broke away from his battle with Ace to kill Luffy, Ace shielded his brother with his own body, resulting in his death. As Luffy escaped Marineford, Akainu severely wounded him, giving Luffy his iconic X-shaped scar on his chest.
Luffy definitely has a score to settle with Akainu, to the point that most of the One Piece video games include Akainu as the final boss, but with how unpredictable One Piece tends to be, just because this is an obvious match up doesn't mean much.
Lastly, there's Buggy the Clown. Buggy was one of the first enemies Luffy fought in One Piece, making his debut all the way back in Chapter 9. Buggy's story has shown to be a parallel to Luffy's. While Luffy is strong, courageous, and simple-minded, Buggy is weak, cowardly, and devious. Luffy has fought his way through harrowing situations, forging a path made with pure willpower alone, while Buggy is a perfect example of someone who falls upwards.
After aiding Luffy in his infiltration and escape from Impel Down, the highest security prison in the world, Buggy gained an enormous following during the Summit War when it was revealed he was a member of the Roger Pirates. During that same war, he survived an encounter with the world's strongest swordsman, Dracule Mihawk, based on the simple fact that his devil fruit, the Bara-Bara no Mi, allows him to split his body into pieces, making him immune to slashing attacks.
Following the war, he was invited to join the Seven Warlords, a group of government endorsed pirates, further spreading his influence. Buggy used this influence to create a delivery company, giving him a foothold in the Underworld, a black market. After Luffy defeated another Shichibukai, Donquixote Doflamingo, Buggy's power in the Underworld grew.
After the defeat of Big Mom and Kaido, two of the Four Emperors of the Sea, Buggy became one of the new Emperors. In the latest chapter, it was revealed that Buggy is the leader of the Cross Guild, an organization that places bounties on Marines. Under his command are former Warlords Mihawk and Crocodile, though the details of how this came to be have not yet been shown.
Though unintentional, Luffy's actions directly led to Buggy's rise to power. Without Luffy, Buggy would not have been able to achieve his successes, and would likely still be imprisoned in Impel Down. Their stories are inextricably bound to each other, which makes him a great choice for a final opponent.