There is no lack of confidence missing from Dragon Ball’s Goku, Vegeta, Gas, and One-Punch Man’s Saitama. Yet, for all the faith they have in their abilities, only Saitama can claim to have never been defeated. One reason for his unreasonable knockout rate is location. All of the opponents in his corner of the manga-verse just aren't a competition. Things would be different though if he brought his talents to Dragon Ball's universe.
The challenge for Saitama would be dealing with the ability of all three to transform and change into different and over-powered forms during a fight. Based on Saitama’s record, some of their more basic forms wouldn't pose much challenge. However, overcoming their ultimate forms would either end in Saitama's defeat or give him the incredible battle he’s been craving since becoming a hero.
At present, Goku’s ultimate form is Ultra Instinct. Rather than a transformation, Ultra Instinct is better described as a state of mind where Goku “quiets” the act of thinking and allows his body to simply react. The goal of Ultra Instinct is to extinguish the delayed reaction that occurs in the mind during a fight. Instead, through Ultra Instinct, the thinking is eliminated, and the body simply reacts. The advantage of Ultra Instinct against Saitama is that Goku would be able to not only avoid his patented one-punch, but every other attack he launches against during the fight. That is, with Ultra Instinct, Goku wins by avoiding Saitama’s strikes, then battering him with his own counterattacks.
In contrast, Vegeta’s Ultra Ego transformation is a response to Goku’s Ultra Instinct. Passed on to Vegeta by the God of Destruction, its core goal is the complete destruction of whatever the user is focused on. As such, unlike Goku’s Ultra instinct that seeks to avoid contact, Ultra Ego craves it. With each hit Vegeta takes, his power increases by unleashing his emotional controls and letting him lean into the fight. The advantage of Ultra Ego against Saitama is that not only would it allow Vegeta to take Saitama’s signature punch, but will make him stronger as a result. In effect, this gives Vegeta the ability to give Saitama a taste of his own medicine. Begging the question of if Saitama could withstand his own one-punch attack? Ultra Ego has a major weakness, however. If a user takes too much damage, he can injure himself. But since Saitama never fought past the “first round”, it is unlikely he’d know what to do in an extended fight.
Finally, Gas’ ultimate form is less a transformation and more of a release of restraint that is achieved when his restraining headband is removed. By removing the headband, Gas turns into an unstoppable force of rage. Similar to Ultra Ego, the more Gas fights, the stronger and more vicious he becomes. When unleashed against Saitama, it's hard to see how one punch would be able to take Gas out. It is more likely to enrage him more, thereby making Gas faster and more powerful. While he was in this “beast” mode, Gas handled Goku, Vegeta, and Granolah. As a result, taking on a single One Punch Man does not seem particularly difficult.
While the One Punch Man has faced a variety of opponents and a multitude of powers, Goku, Vegeta, and Gas are not normal fighters. Besides being some of the most experienced combatants Saitama will have ever faced, their power-ups can be lethal.
