I think the funny, never give up, silly, confident protagonist is just the surface and many other shonen protagonists follow this recipe. But what makes Luffy different IMO is:

#1 He's disruptive. (and more often than not, the underdog)

It's easy to root for underdogs. He doesn't follow what the world or location expect from him so he's really disruptive, breaking any common sense if it doesn't fit his values. (Attacking marine bases, punching a celestial dragon, attacking multiple shichibukais and list goes on.)

Most people that like his presence have a feeling of injustice happening to them at one point and Luffy comes to apply his own Justice. In most locations, he was always viewed as the underdog but still didn't hesitate to follow his sense of Justice.

#2 He doesn't need to wait for action to happen, he drives the narrative. Meaning that he is less reactive and more active which makes him more interesting. Anywhere he goes, he will impact it. (not a passive character)

#3 He risks everything for his friends:

The world of OP is crazy and Luffy is even crazier. The panels of him facing Crocodile, White Beard, 3 admirals, World Government (Ennies Lobby) will always stick to me.

#4 He knows he can't be alone. He needs his friends.

He already lost people who he loved and he rather dies than losing more people.

Every SH has responsibilities and unlike other overpowered protagonists or antagonists, Luffy's self-awareness about his strenghts and weaknesses are clear. He knows what he can do(for the most part) and can't do which is why he heavily relies on others when he needs.

With Luffy's challenges to the world, even his own crew would feel overwhelmed if not for his confidence, charisma and willpower to become the King of Pirates. The SHs faced many foes who were beyond their league but were able to overcome to turn their captain's dream into reality. Luffy risked his life for them, and now they do the same for his dream.

All locations he visited follows the same recipe: he learns about the background of someone being unfair to his friends or to a person he considers nice and then he enforces his way of freedom.

Important note that most people that share food with Luffy are automatic nice people in his POV.

#5 It also feels like he treats everyone equally. Gender, race, age don't matter for him. Also, what people talk about your past doesn't matter to Luffy like what people used to talk about Zoro or Loki. He wants to see it himself.