Monkey D. Luffy is the protagonist of Eiichiro Oda's One Piece manga. Hailing from the East Blue, Luffy leads the Straw Hat Pirate crew and aims to become the King of the Pirates. Over the years, Luffy has become one of the strongest people in the entire world and much of that is down to his Devil Fruit ability.
Using the powers of the Gomu Gomu no Mi, Luffy has become a rubber person and his body allows him to access certain powers, known as Gears, when he needs them. With time, Luffy has developed Gears that make him strong enough to fight even the top-tiers of the verse on an equal footing.
6 Gear Second
Gear Second was created by Monkey D. Luffy during the Water Seven Saga of One Piece and was also the very first gear that he came up with. It debuted during Luffy's fight against Blueno on the Tower of Law. Luffy came up with this after seeing the members of the CP9 use their feet to move extremely fast.
Although CP9 had years of training to pull this off, Luffy used his Devil Fruit powers to pump blood faster in his body and thus dope his body, giving him a surge of energy. Usage of this power also makes his sweat evaporate which gives the impression of steam coming off of his body. This power makes him incredibly fast and powerful, however, in terms of physical strength, it is certainly the weakest that Luffy has right now.
5 Gear Third
Also learned by Luffy during the same saga, Gear Third is another technique that drastically boosts Luffy's power. To use this gear, Luffy bites his thumb and pumps air into his bones, making them huge and thus turning his limbs into those of actual Giants of the One Piece world. Expectedly, all the attacks that Luffy has in his Gear Third arsenal hit much harder than any average attack would.
However, instead, he ends up sacrificing his speed. After the time-skip, Luffy overcame this drawback as he learned to control Gear Third better. At the same time, he also managed to avoid shrinking after using this power. Gear Third, while strong, is still not Luffy's most impressive weapon.
4 Gear Fourth: Tankman
Gear Fourth is the strongest Gear of Luffy that he invented during his time at Ruskaina, the island of 48 seasons. There, under Rayleigh's guidance, Luffy managed to come up with a stronger technique, known as Gear Fourth. Tankman is one of the many forms of Gear Fourth that Luffy has developed.
As the name suggests, this form of his focuses on defense and can tank a great deal of damage. In return, however, Luffy sacrifices his mobility as he isn't able to move too much in this form due to the mass. Interestingly, Tankman hasn't been used in the story so far, however, another version of it, known as Gear Fourth: Tankman - Stuffed Version has debuted during the Whole Cake Island arc.
3 Gear Fourth: Tankman - Stuffed Version
This form of Gear Fourth was used by Monkey D. Luffy during his fight against one of the Three Sweet Commanders of the Big Mom Pirates, Charlotte Cracker. During this fight, Luffy ate a tremendous amount of biscuits, thanks to Nami's help, and became massive in size.
Not letting this power go to waste, Luffy used Gear Fourth and entered the Stuffed Version of Tankman. As expected, this form packed a much greater defensive capability since Luffy was much bigger in size. It was strong enough to tank a direct attack from Charlotte Cracker and one-shot him with his counterattack, called Gomu Gomu no Cannonball, sending him miles away and knocking him unconscious for days.
2 Gear Fourth: Snakeman
Gear Fourth: Snakeman is one of the most impressive forms of Luffy that he used during his fight against villain Charlotte Katakuri, another one of the Sweet Commanders of the Big Mom Pirates. Unlike Tankman, this form of his focuses more on speed and was designed to counter people with incredible Observation Haki.
Even someone like Charlotte Katakuri, who could see the future with his Observation Haki, had trouble keeping up with its speed. All of Luffy's attacks in Snakeman are ranged and hit the target in an unpredictable fashion. The weak point of this form is that it sacrifices the extra defense that Gear Fourth offers for additional speed.
1 Gear Fourth: Bounceman
The strongest form of Luffy's Gear Fourth so far remains to be Bounceman. This technique debuted during the Dressrosa arc when Luffy fought Doflamingo and since then, has become his go-to Gear Fourth form. Although it isn't as tanky as Tankman or as fast as Snakeman, it manages to find the right balance between the two.
Bounceman makes use of elasticity that comes from the heavy contraction of rubber due to Haki and thus launches attacks carrying great power behind them. It also allows Luffy to fly using the same principle, giving him yet another dimension during fights. Bounceman is incredibly powerful and its raw power was enough to send even Kaido flying with a Kong Organ, although the Yonko took no damage due to Luffy's weak Haki.