Saitama is the main protagonist of One Punch Man. From the very first episodes of the anime, it was extremely clear that Saitama was freakishly strong. With his level of power, Saitama can defeat his enemies as if he were swatting a fly.
Saitama's high power levels mean that most of the fights that he is involved in end too quickly. While this is a demonstration of his power levels, it affects the balance of the story, as no one wants to see every fight conclude in a few seconds. Thankfully, the One Punch Man writers have handled this problem exquisitely by adding more substance to Saitama's fights.
6 Vs. Carnage Kabuto
Carnage Kabuto was a mutant created by Dr. Genus. The monster possessed tremendous physical strength and speed, which made him a dangerous opponent. Genos tried his luck at fighting him, but he was easily destroyed by the monster. With Genos in no shape to compete, Saitama had to step in. Carnage Kabuto kept on talking, and for the most part, Saitama ignored him.
However, once the monster mentioned that Saitama would be stuck in the same place until the next Saturday, the hero lost his cool. He realized that he could not afford to stay for that long and ended up defeating the Dragon-level threat with a single punch. While Saitama has definitely fought many stronger enemies, this fight reminds the fans that he will always be the simpleminded hero that they adore.
5 Vs. Genos
Genos is the biggest Saitama fanboy, and he is willing to defend his master at all costs. Prior to his transformation into a fanboy, Genos was only focused on becoming stronger, which is why he wanted Saitama to teach him. Being the lazy person that he is, Saitama refused to train him at all costs.
However, Genos wasn't ready to give up on his goal. After being pestered repeatedly, Saitama finally caved in and decided to have a training bout with Genos. From the get-go, Genos went all out in an attempt to gauge the difference in their power. Saitama was only focused on dodging Genos' attacks, but the latter stated that he wanted a fair battle. Saitama obliged, and, in an instant, he created an opening and launched a punch at Genos, only to stop it before it could hit his pupil. When Genos turned back to see the damage caused by Saitama's attack, he was baffled to see the mountains being destroyed.
4 Vs. Deep Sea King
The Deep Sea King was the main antagonist of the Sea Monster arc. He was the ruler of the sea folk, a race that originated from the sea. As their sovereign, he was the strongest member of the entire race. He had far greater physical strength than ordinary humans, and he also had extraordinary regeneration.
The Deep Sea King's encounter with Saitama is quite memorable, as it wasn't just about defeating the monster. Saitama's victory over the Deep Sea King showed another aspect of the hero's life. The bystanders who saw Saitama kill the monster believed that it was a fluke and that he couldn't have pulled it off if the other heroes hadn't weakened him. The abuse that Saitama suffered shows that even the strongest are not always treated fairly.
3 Vs. Boros
Boros came to earth in order to find a worthy opponent for himself. Ages ago, Boros learned of a prophecy about finding a strong opponent on the planet inhabited by humans. Boros called himself "Dominator of the Universe," as no one had been able to stand up to his might.
Boros' power was on a completely different level from any opponent that Saitama had faced till that point. The two fighters were involved in an epic battle that caused widespread destruction. After witnessing Saitama's power, Boros was confident that he was the man mentioned in the prophecy. So, the extraterrestrial decided to go all out, risking his life to defeat Saitama. However, Boros still fell short. Unlike his previous opponents, Saitama showed Boros a great deal of respect and even lied to give him peace.
2 Vs. Orochi
Orochi was the leader of the Monster Association. He was a ruthless character who had no qualms about killing his opponents. As a Dragon-level mysterious being, Orochi had many different abilities at his disposal. He was able to alter his body shape, manipulate fire, regenerate, and absorb energy from the earth.
Orochi sensed a great deal of power emanating from Saitama, and in order to confirm his suspicion, he decided to test it. Needless to say, the monster was surprised by the results. As the fight progressed, Orochi took Saitama further underground. The villain absorbed energy from the earth's core and launched a powerful attack at Saitama, who simply canceled it out with his own. Shortly afterward, Saitama defeated Orochi with a single hit.
1 Vs. Awakened Garou
Saitama has always been looking for an exciting challenge, and his dream finally came true when he fought against Garou. Normally, Garou would not have given Saitama anything to think about, but after receiving powers from God, the Hero Hunter changed.
Garou's power reached an all-time high, and he was able to trade blows with Saitama for quite some time. The intensity of their punches was considered to be dangerous for the entire planet, so the two had to be shifted to another place where they were free to run amok. Eventually, Saitama managed to get the better of Garou and put an end to his madness. The fight was filled with emotions, powerful attacks, and widespread destruction, making it one of the best fights in the entire series.