Whether it's Goku or Naruto, Luffy or Boruto, the hidden genius of One-Punch Man is that once a reader comes to understand the sheer invincibility of Saitama it's almost impossible to avoid comparing him with other heroes. Naturally, discerning minds don't think alike, and the debate over who can beat One-Punch Man keeps Saitama at the forefront of any discussion on current manga. In Boruto's current Code story arc, however, there is a character who would have no problem absolutely dismantling Saitama in a fight.
Daemon is a member of Kara, a secret organization established by Isshiki Otsutsuki's vessel Jigen for the ultimate purpose of cultivating a God Tree for the glory of the Otsutsuki clan. Daemon was once a human child but subsequent to his membership in Kara was biologically modified with advanced ninja tech that, makes him power and skills that belie his age and child's body.
While the full extent of his powers has yet to manifest, what is known of his abilities makes it clear that Saitama wouldn't stand a chance. Daemon was one of a handful of Kara cyborgs who Jigen ordered decommissioned on account of their powers, which even he was afraid he would not be able to match once they unlocked their full potential. First, there are his base physical abilities of speed and strength. After Jigen's order to destroy Daemon and his sister Eida, rather than obey the order, Boro put them in cryostasis instead. Within a few minutes after being revived. Daemon was operating a full capacity, as illustrated by his ability to sneak up on Code and dispatch a team of highly trained guards in minutes. This is not to say that One-Punch Man would not have trouble taking down a super-cyborg but it does illustrate that physically he is no slouch.
Much more worrying for Saitama though, is Daemon's reflection power. Daemon has the ability to reflect any attack launched against him back at the attacker. So, when one of the security guards attempted to decapitate Daemon, he ended up decapitating himself. The more aggressive the attack, the more aggressive the reflected attack will be against the opponent. Moreover, the ability does not require an actual physical action to manifest. An attacker's "intent" is enough to trigger Daemon's ability to turn that intent to harm him against the persons with the intent. In this way, Daemon is invincible, at least from any physical attack, because any such attack will be turned against the opponent. Daemon's reflecting power is so dangerous that even his sister Eida admits that it can overcome her own ability to see and understand everything from the time she was born to the present. Even when an opponent chooses to not attack, Daemon can then press his attack without fear of retaliation.
So how does Daemon's reflection power stack up against Saitama's one-punch? It absolutely dominates it. Saitama has beaten opponents using a variety of methods including speed, surprise, and power. That is, as long as he puts himself in a position to land a punch, he has never lost. Saitama has even been able to overcome psychic attacks such as the one he encountered when he fought Geryuganshoop.
However, Daemon's reflection power is not psychic. Boruto shows that he cannot make people fight themselves. He can only reflect their attacks back. If they do not attack him, or think of attacking him, Daemon has no power over them. But once One-Punch Man's Saitama thinks about hitting Boruto's Daemon, he'll literally one-punch himself into oblivion.