Saitama can defeat anyone in his universe with a single punch, but what about all of those characters that lie outside his jurisdiction?
Saitama is the protagonist of One-Punch Man, best known for his physical prowess. His uniqueness lies in his punch, he can defeat his enemies with a single punch, hence the name One-Punch Man. Many times, Saitama has been epitomized as one of the strongest beings not only in One-Punch Man but also in a cross-comparison between other anime characters.
Whilst there is no doubt about the fact that Saitama possesses absolute power, he can punch worlds and galaxies and whatnot, exaggerations like this are not unknown in anime. There's Saitama of course, but there are other characters in different series who possess unlimited powers (Naruto, Goku) and there are less powerful beings than Saitama too.
10. STRONGER: Goku, Dragon Ball Z
For one thing, Saitama can't really defeat everyone in One-Punch Man. Goku comes from Dragon Ball Z, known for his penchant for fighting. One of the things in the Goku vs Saitama debate is that the latter is a gag character, so it is pointless and it doesn't matter who he is up against. But truth be told, Saitama did blow away a storm by only punching it, so that's not a reason enough.
Comparatively, Goku's physical feats and abilities are far stronger than Saitama's. He's obliterated the walls of an alternate dimension and shook a universe by just arriving in it.
9. WEAKER: LORD BOROS, One-Punch Man
Boros, aka Lord Boros, comes from the same franchise of One-Punch Man. His and Saitama's stories parallel, in that both of them went through a self-imposed existential crisis. Lord Boros was eventually obliterated by Saitama in the battle popularly known as Saitama vs. Boros. Saitama wiped out Boros by unleashing his Trump card: 'Killer Move: Serious Series, Serious Punch'. The battle took place after Saitama defeated Geryunganshoop and broke into Boros' throne.
From shattering Boros' armory with a punch to taking a few blows himself, the battle was a real nail biter. Boros put up a good fight by unleashing his destructive energy at Saitama, but it got nearly impossible to harm the latter. In the final showdown, Saitama obliterated Boros.
8. STRONGER: Giorno Giovanna, JoJo's Bizarre Adventure
The fifth JoJo of JoJo's Bizarre Adventure, Giorno Giovanna's stand out ability is the 'Gold Experience Requiem' which he attains after he pierces Gold Experience with Stand-creating arrow. With this warping ability, he can nullify his opponent's willpower and actions to effectively zero. Hence, it doesn't really matter how strong Saitama is because he can't fight reality warping.
Giorno's physical ability has a slightly different magnitude, in that Giorno can practice a Stand, i.e. the skill to create and manipulate life. This short-ranged Power type ability can let Giorno unleash a timely and abrupt barrage of lethal punches on Saitama, thereby significantly harming him.
7. WEAKER: Hinata Hyuga, Naruto
Hinata is the heiress of the Hyūga Clan from Naruto. Due to Hinata's initial failure to cope with her training routine, she was ousted from inheritance by her father, Hiashi. But that didn't stop her, she joined Team 8, trained hard and became stronger to the point, where Naruto himself had to acknowledge her strength.
However, Hinata's best abilities are chakra prowess and control, Byakugan, Taijutsu and medical ninjutsu. She wouldn't stand a chance in raw strength against Saitama's superhuman strength such as that of jumping in and out into outer space, etc. Saitama shows no signs of fatigue, whereas Hinata grows weary often.
6. STRONGER: Broly, Dragon Ball Z
Pitting Saitama against Broly is pitting One-Punch Man and Dragon Ball Z against each other. It would represent an epic showdown. Broly has obliterated planets and stars in the span of his career.
Broly has tanked a Kamehameha at Point Blank without any damage. In his restricted state he destroyed the South Galaxy which comprises 1/4th of the Dragon Ball Macrocosm! Sure Saitama can wipe out planets with a Punch but can he even breathe in space?
5. WEAKER: Mr. Satan, Dragon Ball Z
This should be an easy match, Mr. Satan isn't the strongest in Dragon Ball Z, there are countless times when he has ditched people. Hercule/ Satan is nothing but a scam. But in his early years, Mr. Satan was a top-notch martial artist in his own right.
For a normal human, Mr. Satan is incredibly strong. He's the butt of jokes in Super but and beating One-Punch Man is another ball game altogether. The battle would be between a former martial artist and a man who punches storms, so no points for guessing.
4. STRONGER: Sōsuke Aizen, Bleach
Sōsuke Aizen comes from Bleach. As the former Captain of Gotei 13, a division of Imperial Guards, he possesses unmatchable agility and dexterity that lets him finish his opponents in no time. Sōsuke Aizen is also an ace swordsman who defeated Ichigo Kurosaki. Not to mention Aizen's Reiatsu control that he can use to shield himself and vaporize anyone who comes close to him.
On account of Aizen's strength, swordmastery, Reiatsu, and hypnosis, it is nearly impossible for Saitama to defeat him.
3. WEAKER: Izuku Midoriya, My Hero Academia
Between the young protagonist Izuku Midoriya of My Hero Academia and Saitama, the latter is the obvious choice. But that is not to say Izuku is weak. Saitama possesses a greater set of physical abilities, he has punched a Titan, stars, planets, and whatnot.
The young Deku/ Izuku can only use 5-20% of the full cowling i.e. his ability to use reflexes, maneuvering and greater strength. Saitama uses only 0.01% of his power when it comes to a Serious Punch and leaves some serious obliteration in his wake.
2. STRONGER: Naruto, Naruto
Naruto's speed has passed the speed of light and there isn't a possible way for Saitama to defeat that. Naruto's durability is showcased when he survived a planetary explosion at point-blank range. Saitama may get full marks for putting up a good fight since he undoubtedly is stronger of the two.
The problem lies with Saitama and Naruto's similar abilities: One-Punch and Rasenshuriken (wind release Jutsu), respectively. Naruto wins by virtue of his stamina and speed. If you are faster than your enemy, it tilts the battle in your favor. Not to mention how Naruto can KO Saitama with an extreme Jutstu.
1. WEAKER: Mumen Rider, One-Punch Man
An easy choice, it would be fairly simple for Saitama to knock the wind out of Mumen Rider, a regular guy without any special skills or abilities. He is just a C-Class Rank 1 professional hero of the Hero Association in One-Punch Man who fights opponents above his abilities.
All of Mumen's justice combats, such as tackle, hand to hand, crash, crane high kick will be rendered useless in front of the insane physical prowess of Saitama. His immeasurable Agility, Stamina, Senses Dexterity are extremely resistant for Mumen to match.