Since the emergence of the One-Punch Man anime in 2015, Saitama's incredible might has been the subject of conversation and comparison. "But can he beat Goku?" is a question often thrown into the mix whenever there's discussion about a powerful anime character, but that might be a misguided question in this case.
Who'd win between Saitama and Goku is definitely an interesting question; however, it may not lack a definitive answer. A better question, one that may be able to go further could be whether or not Saitama can do something Goku hasn't, and defeat Arale Norimaki, a character from Akira Toriyama's gag manga, Dr. Slump.
Arale Norimaki
Arale is an android built by the scientist Senbei Norimaki. She is energetic, naïve and completely lacks common sense. Arale is also ridiculously strong, doing things like accidentally destroying cars when running out into traffic. Arale is a near-perfect android, with her only flaw being her near-sightedness. Arale is subject to something some people have called "Toon Physics", in which things that would regularly be fatal are merely brushed off, and various impossible feats are made possible for the sake of making a joke. Her stats are insane, like being able to destroy Earth with a single punch, walk at Mach 1 speed, and various other unfathomable skills.
In Dragon Ball Super, Arale made a cameo appearance in episode 69, where she went head-to-head against Vegeta and Goku one after the other. She completely embarrassed them both, but Vegeta made a key observation: Arale operates on an entirely different set of rules – the rules of gag manga. After applying the ridiculous illogic of gag manga to his combat, Vegeta was finally able to land a blow on Arale. It is for this reason that Arale vs either Goku or Vegeta is an unfair match-up, and by extension, pitting Saitama against a similar set of characters brings up that same incongruency.
To better illustrate Arale's impossible strength, she matches a Kamehameha fired by Goku in his Super Saiyan Blue form with a ki blast of her own, before following the resultant beam struggle with another ki blast 100 times the power of the last. This stumped Goku, who was happy to meet her again, saying that she's just as strong as he remembers. Essentially, Goku has never actually beaten Arale, and the only character she officially couldn't beat is another Dr. Slump character subject to the same forces.
Saitama
Like Arale, Saitama is the protagonist of a gag manga and possesses incredible power, as the series title would suggest. Due to his power being an inherent quality of One-Punch Man as a narrative (sequential or otherwise), Saitama's immense strength is also by definition an aspect similar to Toon Physics as far as its consequences go. Like Arale, Saitama is unbeatable, apparently impossible to damage, and overcomes any opponent with relative ease. Like Arale, Saitama faces a few strong characters from his own universe, but unlike her, Saitama has never been incapable of defeating another character from the One-Punch Man universe.
The Special Position of Arale
Both characters are defined in their respective universes as characters with unmatched strength, operating according to the rules of gag, the ambiguous and amorphous realm where characters limitations are delightfully inconsistent or impossibly the opposite. Gag anime characters like Saitama and Arale operate in their own distinct metaphysical category, where the very nature of their being is irreconcilable even if they exist in universes that are filled with elements that operate in their reality in similar ways to them, to similar degrees. Arale does not necessarily defeat every opponent in a single attack like Saitama does; but she is capable of planet-destroying feats and incredible strength that Goku struggles to keep up with, at least as recently as Super Saiyan Blue. The air pressure from Saitama's punches (the ones that don't make contact with anything) is powerful enough to level cities, and it has been said by series manga artist, Yusuke Murata, that Saitama could destroy the planet Earth if he so wished.
These two are perhaps very well-matched characters subject to the physics (or lack thereof) in the realm of gag that they inhabit, and also in the context of the permanence of comparisons to Goku when it comes to strong characters. What gives Arale relevance in this case is the fact that while fans can't find a definite answer regarding who'd win between Saitama and Goku, there's been evidence that at different points in his life, the benchmark of strength that is Goku has never been able to have a conclusive victory against Arale. Who would win between Saitama and Arale is hard to say given their incredible strength and durability, but perhaps this question can bring fans a step closer to deciding who they think would win between Saitama and Goku. This is because of Arale's unique position of having inhabited a universe other than her native, consequently canonically existing in both gag and non-gag media; as well as being a character who has faced Goku multiple times without losing even once.