Below are 9 characters who are opponents, but they are also the ones who help Luffy `grow stronger`
1. Arlong fishman pirate
Arlong was one of the first `tough` opponents Luffy encountered when he first set sail.
Although Nami had joined the crew before, it was only in name, she wanted to take advantage of these stupid pirates to earn more money to buy back the island invaded by the Arlong crew.
2. Crocodile Shichibukai
As a former Shichibukai recognized by the World Government, Crocodile’s strength is `not average`.
On the other hand, before being defeated by Luffy, Crocodile had defeated Luffy at least twice, so he was also the first opponent to make Luffy taste defeat.
3. God Enel
God Enel is an extremely dangerous enemy of Luffy on Sky Island.
Luckily for Luffy, he ate the rubber fruit, the natural enemy of the lightning devil fruit that Enel ate because rubber does not conduct electricity, so he won a spectacular victory after being left alive.
4. CP9’s Rob Lucci
The battle between Lucci and Luffy at Enies Lobby was an extremely satisfying battle between two equally talented opponents.
However, thanks to the encouragement of his teammates and the spirit of steel, Luffy stood up and used all his strength to punch Lucci away, causing him to lose consciousness and be `knocked out`, winning a spectacular victory.
5. Warden Magellan
General Dementor Magellan possesses the power of the Paramecia Doku Doku no Mi devil fruit, so he is also known as Poison Man.
If it weren’t for Bon Clay risking his own life to go down there to rescue Luffy and then bring him to Ivankov for timely treatment, Luffy’s `little life` wouldn’t have been able to be kept.
6. Mad Scientist Caesar Clown
Caesar Clown is a mad scientist, rival of the world’s number 1 scientific genius Vegapunk.
But for Luffy, it’s different, because he is immune to toxins after his encounter with Magellan, so much of Caesar’s poison gas is a small matter.
7. Hordy Mermaid Pirate
Hordy Jones – a mermaid of the great white shark species and former captain of the New Fishman Pirates.
The battle with Hordy was Luffy’s first fierce battle after the 2-year timeskip.
8. Shichibukai Don Quixote Doflamingo
Doflamingo is the toughest opponent Luffy has encountered.
Doflamingo is Luffy’s first opponent to possess both Armament Haki and Conqueror’s Haki and can awaken the power of the Mera Mera no Mi devil fruit.
Who knows, after witnessing Dof’s devil fruit awakening, in the near future Luffy can also awaken the Gomu Gomu no Mi devil fruit?
9. Ice sweet commander Bigmom – Cracker
Cracker can be considered the strongest character that Luffy has ever defeated.
In Gear 4 Tank Man state, Cracker’s sword cannot pierce Luffy’s body but instead sinks in and is knocked back very far.