Contains spoilers for One-Punch Man chapter 169
While Saitama is without a doubt the strongest character in One-Punch Man, he is no longer the most important one. After the eventful conclusion of the battle against Garou saw Saitama travel back in time, the sole person who knows of this is his disciple Genos, who is the only one aware of the existence of alternate timelines, and also that his master and best friend did the impossible to save everyone.
The biggest fight in One-Punch Man history ends in a way that fans could have never imagined. After being overwhelmed by Saitama, Garou realizes that he's accidentally killed the bystanders, including Tareo, a kid who has been his only supporter. Garou regrets his actions and begins to be consumed by God, the mysterious villain who granted him his last power-up. Before dying, he asks Saitama to copy his final technique, which allows him to travel back in time. The bald hero sees the events of the past unfold in front of his eyes, and at the right one he "jumps in" and defeats Garou, literally punching God's power and nefarious influence out of him with a punch. This effectively erases the future and saves everyone who is killed by Garou. However, "future" Saitama then merges with his present self, who has no memory of that diverging timeline.
There is, however, someone who remembers everything that happened. In chapter 169 of One-Punch Man, Saitama rejoins Genos, who had been killed in front of his eyes in the future timeline. "Future" Saitama has brought back with him Genos' core, the computing unit that stores the cyborg's memories. When the Genos from the present connects his body to the future Genos' core, he is able to learn everything that happened in the erased future. Considering that neither Saitama nor Garou have any memory of that, this makes Genos the only person in the world who is aware of the existence of a One-Punch Man's multiverse of infinite possibilities and alternate realities. He is also the only one who knows that Saitama has once again saved everyone, without taking any credit (this time because he doesn't remember).
Genos is, without a doubt, the most important person in Saitama's life. When he sees his friend and self-proclaimed disciple die, the hero snaps for the first time ever, his anger causing him to reach even higher levels of power. This also makes Saitama question his right to call himself a "hero" because he is unable to be there for his friends when it matters. Genos represents Saitama's link to humanity, the anchor that keeps him from losing his sanity under the weight of his own immense strength. Now, he is also the only one who can tell the world how amazing Saitama actually is.
Through the loss of Genos, Saitama evolves for the first time as a character and as a hero. Even if the conclusion of the battle apparently brings everything back to "normal", Genos' awareness of the alternative future timeline acts as a reminder, for the readers and for the other characters in the series, that Saitama is indeed the greatest hero in the universe, or actually in the multiverse. The long story arc that concludes in chapter 169 makes Genos the most important character for Saitama, and hence for the whole One-Punch Man series.