Monkey D Luffy is the protagonist of One Piece. He tends to be relatively carefree and yearns deeply for great adventures. With that free spirit, Luffy aims to become the King of the Pirates, a lofty and ambitious dream only ever achieved by the previous Pirate King, Gol D Roger.
Luffy seeks this pinnacle of piracy, not for the fame nor the fortune, but so he might become the freest man in the world; a man strong enough to proudly live his life on the terms that he dictates.
10 CAN'T DEFEAT: SUPERMAN
Superman is the quintessential powerful superhero; it's literally in his name. He is known for his myriad of abilities including, but not limited to, super speed, super strength, heat vision and frost breath. Luffy has a natural immunity to blunt force attacks so that would greatly aid him in a battle aganst the Man of Steel.
Despite this, however, Superman's arsenal has a lot more variety, so he could easily counter with his different elemental type abilities.
9 CAN DEFEAT: NIGHTWING
Dick Grayson was the first sidekick of Batman, operating under the hero name, Robin, before venturing out on his own and earning fame as the hero, Nightwing. Grayson is one of the coolest characters you will come across in comics, due to a mixture of his brainy personality and his bad boy nature.
In his younger years, Grayson was a part of the circus and the agility he developed in that time plays a major role in the current fighting style he uses. Nightwing has pushed himself past the peak of human abilities but, unfortunately, he would lose to Luffy who has already gone far beyond that level.
8 CAN'T DEFEAT: HULK
Bruce Banner or the Hulk, as he is more commonly known, is a scientific genius who subjected himself to Gamma radiation and gained the ability to transform into a powerful green beast. The Hulk is commonly considered to be one of the most powerful characters in all comic and he is believed to have an unending potential for growth.
This is due to the fact that the Hulk manages to become stronger whenever his anger level increases. Luffy developed his gear 4 techniques in order to combat gigantic monsters, which makes it ideal for fighting against the Hulk but, ultimately, this would prove a step too far for him to get past.
7 CAN DEFEAT: SPIDER-MAN
Spider-Man is a friendly neighborhood hero who does his best to come to the rescue of the troubled New Yorkers in the Marvel universe. The origin tends to change, but Peter Parker is a human with spider-like abilities and he donned a costume in order to become the great hero that his people needed.
His abilities means Luffy would have a tough fight against him, but luckily for Luffy, he is quite used to taking down spider-webs. As such, it could be expected that Luffy would beat Spider-man in a similar manner to how he defeated Donquixote Doflamingo.
6 CAN'T DEFEAT: AQUAMAN
Aquaman is a half human, half Atlantean warrior and the all-around King of Atlantis. He has several abilities, inclusive of the power to communicate with and control marine animals, super strength, enhanced speed and the ability to survive both on land and in water.
In many ways, Aquaman is a natural enemy to Luffy, as he is the true King of the Sea. Any fight between the two on the sea would be incredible hard for Luffy, due to his inability to swim after consuming his devil fruit, meaning, Aquaman comes out as winner.
5 CAN DEFEAT: BEAST BOY
Garfield Logan is a consistent member of the Teen Titans, where he regular goes on dangerous missions to save the lives of civilians. As a hero, Beast Boy tends to be carefree and relaxed in a similar manner to Luffy. Garfield is a superhuman with the ability to tranform into any animal he pleases.
This gives him a very broad arsenal which can pose a problem for most opponents. Unfortunately for Beast Boy, Luffy has spent much of his life battling against numerous animals, which means he'd be prepared for whatever Beast Boy could throw at him, eventually lead to Luffy's victory.
4 CAN'T DEFEAT: THE FLASH
Barry Allen (aka The Flash) became the fastest man alive after he was struck by lighting and dosed in a vat of mysterious chemicals. This unlikely string of events connected Barry to the speed force and, with that connection, he became a speedster.
After years of further research and use of his powers, Barry learnt more about the speed force and this allowed him to go even faster than before. Flash is widely regarded as the fastest person in comics and this speed, mixed with his power, means he could defeat Luffy.
3 CAN DEFEAT: STAR LORD
In 1988, Peter Quill was abducted from Earth and this proved to be the start of a very long journey that would eventually turn him into one of the Guardians of the Galaxy. As the name suggests, Peter and his team are responsible for traveling around the galaxy, helping those in need, whilst bringing intergalactic criminals to justice.
As a Celestial-human hybrid, Peter is stronger than normal humans, with abilities such as enhanced durability and power. Despite this, Star Lord would surely lose against against a Gear Fourth Luffy, if he were to ever become an intergalactic criminal and the two were to have a brawl.
2 CAN'T DEFEAT: BATMAN
Bruce Wayne is easily one of the most iconic names in comic book culture, having donned the mask of Batman from all the way back in 1939. Since then, he's been kicking the collective butts of his personal Rogues Gallery, as well as the many other villains that plague the DC universe.
Of course, Bruce has no superpowers, but his intellect and weaponry are often more than enough to take on rampaging, super powered evil-doers. Bruce's intellect means that he could easily find a way to exploit Luffy's devil fruit's weakness, and Luffy's lack of intellect means he would have a hard time preventing this from happening.
1 CAN DEFEAT: ANT-MAN
Ant-Man is an ant themed hero, who can drastically alter the size of his body to microscopic or gigantic levels. Of course, the ability to become much larger in size is an ability common to Luffy as well. Luffy can temporarily increase the size of his body parts by blowing air into his rubber bones and muscles.
This means Luffy can compete with Ant-Man should he try to rely on power. If Ant-man chooses to reduce himself in size, he would then become an easy target for Luffy, who could defeat him using a future sight to locate Ant-man before finishing him off with an armament enhanced attack.