As a B-Class Hero in One-Punch Man, Saitama is not expected to indulge in any serious fighting, even though the Hero Association is wary about his real strength level. That being said, it doesn't matter what anyone else thinks, because Caped Baldy is hands down the most powerful superhero in the world, at least as far as the story has reached until now.
Ranking his best fights — in the anime or otherwise — is a surprisingly simple task. The ones chosen are not based on irrelevant features such as spectacle (there are enough of those), but on how they help progress the narrative in a more or less cogent direction. In Saitama's perspective, however, almost none of these battles matter, since they end at the same instant that they begin.
10 Vs. Vaccine Man
The first fight shown in the series occurs between Saitama and the massive purple-monster calling himself Vaccine Man. The villain effortlessly overpowers Lightning Max and Smile Man, neither of whom could come even close to his level of power.
However, when Saitama appears, Vaccine Man becomes aggressive as the hero couldn't care less about his pollution-themed backstory, and launches an attack. The outcome is surprising to nobody.
9 Vs. Mosquito Girl
Saitama runs after a mosquito, but none of his techniques, as overpowered as they are, have the slightest effect. He later discovers that the villain responsible for the swarm is Mosquito Girl, who seems to be winning against Genos (a previously unknown character).
Luckily for Saitama, the increased size of the target makes the battle a lot easier, and he basically swats her into oblivion. This begins the phase of Genos' limitless admiration for his Sensei.
8 Vs. Speed-o'-Sound Sonic
Saitama's totally minding his own business when a shadow suddenly pounces on him. Speed-o'-Sound Sonic automatically pegs Caped Baldy as a Paradiser, but none of his techniques seem to have the slightest effect.
Furious with his continued failure, he unleashes his iconic Wind Blade Kick, which would have failed even if Saitama hadn't struck the poor ninja straight in the crotch. Sonic threatens to achieve vengeance for this gross insult, but as everyone is aware, this never happens.
7 Vs. Geryuganshoop
Saitama pierces the security measures of the Dark Matter Thieves' vessel as if they were made of paper, and runs around the spaceship looking for whoever's involved in the invasion. He destroys Groribas, alerting the tentacle-y Geryuganshoop to his presence.
The monster proudly avers that he's the greatest telekinetic power in existence, and uses the Gravitational Wave technique which doesn't even faze Saitama. The shower of debris likewise has zero impact but allows the hero to kill Geryuganshoop by bifurcating him with a pebble.
6 Vs. Subterranean King
This sequence is easily one of the most incredible battles in the show, regardless of the fact that it never takes place in real life. A group of Subterranean monsters starts attacking Saitama, but he proves that he is more than a match for all of them put together.
At the end, he faces the Subterranean King, who haughtily announces that the real fight hasn't even begun, exhilarating his opponent to beyond anything he has experienced before. And then Saitama's alarm buzzes.
5 Vs. Garou
Garou has taken down many superheroes at this point, making him overconfident in his own abilities. After his joint defeat of Golden Ball and Spring Mustachio, he starts looking for someone else to humiliate, which is when he sees Saitama hunting for a wig (to masquerade as Charanko).
Garou knocks him with such force that he is pushed into the mud, but the response — a glorious downward chop — results in a complete and utter K.O. The best part is that Garou can barely recall what happened to him when he comes to his senses.
4 Vs. Deep Sea King
Saitama takes a bit too long to reach the scene of action, where the Deep Sea King has already defeated Genos and Mumen Rider, beating one to a bloody pulp and disemboweling the electronics from the other. Unsurprisingly, Caped Baldy tiredly exclaims that he would rather finish it quickly, so he can get out of the heavy rain.
The Deep Sea King, having been interrupted in the middle of his grandiose speech, acquiesces with an earth-shattering punch; however, Saitama merely bores a hole in the creature's abdomen without even blinking an eye.
3 Vs. Carnage Kabuto
In the House of Evolution mini-arc, Genos and Saitama find the scientist, Dr. genus, and his greatest creation, Carnage Kabuto. It is here that the protagonist reveals the secret to his power (a three-year daily regimen of a hundred sit-ups, squats, and push-ups, followed by a six-mile run).
The monster triggers his "berserk" mode, which is supposedly going to extend all the way until next Saturday, a fact that reminds Saitama that he is about to miss a great deal at the local supermarket. He freaks out about this more than having just murdered Carnage Kabuto, until Genos calms him down.
2 Vs. Genos
Genos keeps begging Saitama to have a friendly fight with him, in order to gauge the extent of his sensei's full strength. Nothing works, not even the Demon Cyborg's Incineration Cannons, mostly because he can't even land a hit on his master.
At one point, Saitama feints behind Genos, announcing his victory, but his opponent isn't ready to give up. That is, not until Caped Baldy unleashes his so-called "Death Punch," which evaporates an entire mountain range with just the generated wind force.
1 Vs. Boros
By his own admission, Saitama considers Boros to be the most impressive of all the fighters he has faced so far because only the "Dominator of the Universe" has successfully forced him to reveal his Consecutive Normal Punches as well as his greatest arsenal, the Serious Series: Serious Punch.
The latter destroys half the cloud cover in the atmosphere. The villain's Collapsing Star Roaring Cannon is eradicated, with nothing left except a shattered Boros, thankful that he has finally met his match.