Saitama vs. Crablante

You see, I never wanted to become a salaryman. I wanted to become a hero that blows away villains like you... with one punch. I'm done looking for a job. Bring it on! Saitama vs. Crablante was the fight between Saitama and the Tiger-level monster Crablante.

Prelude
Saitama stands before Crablante, who seems surprised that he is not running in fear like the other people. Saitama sighs and Crablante comes to the conclusion that Saitama is a salaryman and due to the fact that he isn't running, he must want to die. Saitama agrees with that but states that he is, in fact, unemployed and is on a job hunt and that he had already been rejected from a job that morning. Saying that he has lifeless eyes like himself, Crablante decides to spare his life, stating that his real prey is a child with a cleft chin.

While Saitama is walking across the street, he encounters the Split-Chinned Kid Crablante was talking about. He debates on whether or not he should even expend the energy to save him, but once Crablante is in sights and attempts to attack the child, Saitama jumps in and saves the child from his attack.

Battle
Saitama tells the kid to escape, however the child wants to get his soccer ball. Just as he finishes his sentence, Crablante pops the ball with his foot. Crablante is rather surprised that Saitama came to save the kid just after seeing him, and asks him why he saved him. Saitama attempts to calm him down and asks why he wants to kill him. He responds by stating he has killed many more people for a lot less, and shows him the ridiculous nipples he drew on his chest, further describing his aggravation by saying he cannot even remove the markings. Crablante points his claw at Saitama, antagonizing him by saying if he gets in his way, he'll make sure he never goes job hunting again. To his surprise, Saitama begins chuckling out loud, with Saitama saying Crablante reminds him of a villain in an anime he used to watch as a child. Infuriated by his nonchalant comment, Crablante smacks Saitama, sending him flying into a nearby concrete shed. Crablante then focuses his attention back on the child, but just as he is about to end the boy's life, a rock is thrown at Crablante's face. He turns around, seeing a weakened but still breathing Saitama, who tells him he won't let Crablante kill a child with this age of declining birth rates. At that moment, Saitama has an epiphany. He remembers how he always wanted to be a hero as a child, so he could send rotten villains like Crablante flying away with a single punch. Filled with newfound determination, Saitama pulls his tie off his collar and tells him to bring it on. Deeming him a pathetic excuse of a hero, Crablante beings to pummel him around with a series of powerful punches, causing blood to spew from his mouth and forehead.

However, just as Crablante is about to land the seemingly finishing blow, Saitama jumps and propels himself above Crablante using Crablante's claw as a propellant, and wraps his tie around Crablante's right eye. He tugs with all of his might, causing all of Crablante's inner organs to be pulled along with his eye. A torrent of yellow blood then spews from Crablante, while Saitama victoriously stands over Crablante, the first mysterious being he defeated in his life.

Aftermath
This fight is perhaps the most pivotal battle of the entire series. While the battle itself is not particularly earthshaking, it is during this battle that Saitama decides to become a hero and embark on the relentless and bone-crushing training regimen that resulted in his immeasurable strength.

Another direct result of the battle was the creation of the Hero Association. After Agoni, the grandfather of the boy Saitama saved, heard about the incident, he created the organization to gather heroes that could protect society from monsters.

Trivia

 * The battle between Saitama and Crablante lasted 4 minutes and 35 seconds.