While there are several superheroes blessed with the uncanny power of elasticity, two of the more iconic stretchy comic book heroes are One Piece's Monkey D. Luffy and the Fantastic Four's leader Mr. Fantastic. Naturally, since their powers are based on their ability to stretch and contort themselves in unworldly forms, the question becomes, whose stretchability is the strongest.
Neither Luffy nor Mr. Fantastic were born with their powers. Instead, each came onto their powers as a result of an outside event. In the case of Luffy, that event occurred when he ate the "Gum-Gum Fruit" as a child. As a result of eating the fruit, Luffy's body was rendered in a rubber-like state that allowed him to stretch, bend, elongate, and inflate his body. In contrast, Reed Richards' transformation occurred after he piloted an authorized flight into space with his future wife Sue Storm, her brother - Johnny Storm, and Reed's college roommate Benjamin Grimm. The flight, which was conducted without proper protection, was pelted by gamma radiation. That radiation transformed Reed's DNA to give him the ability to stretch every part of his body as if he were a rubber band.
On the surface, it might seem that there is little difference in the stretching ability of Luffy and Mr. Fantastic. Both Luffy and Mr. Fantastic's power of elasticity allows them to reform their bodies into any shape that they wish. In addition, their rubbery consistency makes them immune from most "blunt" attacks such as a punch, kick, or even a bullet. Moreover, their elasticity allows them to transform their bodies into effective weapons. For instance, both can use their bodies to redirect bullets and shells back towards the shooter. In addition, both can use their bodies to wrap up and constrain others from moving. However, a deeper dive into the abilities of Luffy and Mr, Fantastic reveals that not all stretching abilities are equal.
Despite the similarities, there is one crucial metric that separates the top-rated from the runner-up, namely maximum stretch distance. Indeed, how powerful is one elasticity, if one's ability to stretch covers only a short distance. In this respect, Luffy comes out as the clear winner. Indeed, Luffy has the ability to stretch without restriction. That is, he can stretch as far as he deems necessary to accomplish the task. Conversely, while there is no official limit to how far Reed can stretch, it has been demonstrated that there are limits upon which Reed can effectively control his stretching abilities. There is a distance where Reed loses his ability to effectively control these body parts. Conversely, there is no such known limitation for Luffy.
While there is no argument that both Luffy and Mr. Fantastic have incredibly powerful abilities of elasticity, not all rubber-like abilities are the same. The Fantastic Four's Mr. Fantastic might be able to use his powers to wrap up a villain across the street, chances are that One Piece's Luffy can do the same a block or even a city away.