@Ereiam  
Until further notice, Saitama does not possess powers relating to, or resistant too, things like high level magic, psychic powers or reality warping. If he is willed into non-existance, he cannot punch himself back into existence(meaning he was never non-existant) until we see him do it in the source material. We can only take this at face value, if you have to make overt assumptions, predictions and theories based on nothing, then the argument is invalid.
 
For example.
 
ACCEPTABLE  
Character X has so far not been shown to be immune to mind-erasure, therefore a psychic character who can erase minds should be able to erase his mind.
 
UNACCEPTABLE  
I have no evidence that Character X is immune to mind erasure, but he is probably immune to it, therefore he is in fact immune to it by default.
 
Until he shows feats, we can’t just make things up about him like those Dragonball Super losers do about Zen-Oh. Also, we cannot handicap his opponents by denying them their full power or forcing them to act in a certain way(like saying they have to monologue and such like Saitama’s adversaries do)
 
I’m trying to be as polite and rational as possible about this, because I have had dealings with Saitama fanboys before(not accusing you of being one, though) who literally lie, make things up, posit fan fiction and theories as fact, and basically just put their fingers in their eyes and go LALALALALALA when you try to say something to them, so my expectations of any “argument” related to the character is low. It’s a shame, because I love One Punch Man, and the worst fandoms are the ones you actually belong to where most of the other fans are insufferable.
 
Characters like Popeye should be able to beat him on argument of Toon Force and showing superior feats, and a villain like Mandrakk the Destroyer can beat him because he all ready has the full power of Saitama and his whole universe(along with every other fictional universe) by default, and cannot be beaten by anyone besides Superman in the Thought Robot or Super Judge, who had the power to alter the story to take away Mandrakk’s power. Mandrakk literally can only be beaten by breaking the 4th wall.

@Ereiam  
That’d be perfect. Fuck Pinkie Pie vs Deadpool, lets have that death battle instead!

@Evil Emperor Proteus  
You forget one big thing: Saitama is a parody character. His strength has no discernible upper limit and he always easily defeats his opponents as soon as he can reach them (which, due to his poor sense of orientation and limited awareness to his surroundings, can sometimes take a while - which the threat takes advantage of to steamroll every other hero standing on its way - until Saitama reaches it or stumbles upon it, at which point he inevitably curb-stomps it).  
He’s immune to physical attacks, psychic abilities, and I wouldn’t be surprised if he could no sell magic or reality warping as well. Similarly to psychic power, his dense mind also makes him impervious to mind games. When a villain takes too long with his gloating monologue, Saitama’s reaction is to cut them off, either with a flippant demand to get on with it or with a punch (the outcome is the same either way).  
And on top of that, we’re supposed to believe that his abilities come from having followed some mildly intense daily workout routine (the explanation given in the webcomic is a bit more than that, but so far it still boils down to “just because” indeed), which shows how seriously Saitama must actually be taken.
 
If you want to pit Saitama against a character from the Marvel Universe, I think the most sensible match against him would be Deadpool (despite not having Saitama’s overwhelming strength, he has a nigh-unbeatable healing factor, is pretty resourceful, also kind of a joke character himself and enough of an anti-hero to give Saitama probable cause to fight him).  
In any case, Saitama would undoubtedly be eager to fight someone who can take his serious punches and get back on his feet, and Deadpool would probably be curious to see if Saitama can punch him hard enough to break his curse.

@Ereiam  
There are plenty of characters in Marvel who can beat Saitama. One problem with these arguments is that OPM isn’t even finished, so we don’t know everything about Saitama or the nature of his powers. Another is that people just assume the “always wins” has jurisdiction in other settings.
 
Any decent Magic user, Psychic, Reality Warper or Toon Force character should be able to beat Saitama. If Saitama is immune “just because” then I’m afraid Saitama goes in the trash. I’m not saying that’s happened or will happen, but if it does. Additionally, there are just some characters who he literally can’t beat on technicality. For example, he couldn’t beat Mandrakk the Destroyer because Saitama is not a fictional character with enough real world recognition, influence in the real world and just isn’t an inspiring story. Only Superman could beat Mandrakk because of the power and influence of his character and story in the real world and throughout fictionality.

@Background Pony #89FF  
From what I can gather from this wiki page (which doesn’t compare to reading the source material), maybe Saitama would consider that an interesting challenge, then.

@Ereiam  
I’ll just refer you to the Infinity Gauntlet right here

@Background Pony #89FF  
I’m not really familiar with the Marvel universe, but I know that, no matter how ludicrously owerpowered the foe, Saitama’s whole shtick (and personal tragedy) is that he’s always stronger. We’re talking about a guy who took a planet-bursting attack head-on without any damage except to his outfit, and that the strongest esper in the world (one who can throw buildings in the air casually) could hardly lift, let alone harm.

@441TheSecond  
I said WITH the Infinity Gauntlet, not WITHOUT…
 
Lets face it, Saitama can’t really get close to a guy who can erase you from existence with his “powerful as hell” glove…

@Background Pony #89FF  
Saitama one punches him.
 
What part of ONE-PUNCH MAN do you not get?

@Background Pony #89FF
 
Well, with the Infinity Guantlet, Saitama has no chance, but without?

Come to think of it (and this is slightly going off-topic, I know)…what would’ve happened had Saitama faced a guy like Thanos…with the Infinity Gauntlet ready in his hand.
 
A guy who can end threats in one hit VS a guy who can change creation on a whim…

“you monster! you’re putting the esses next to the pees!”

Hmmm i see, might give it a shot in the future when i have some time

@Horsecat  
True, there’s basically no one who can beat Saitaima, but part of the fun of the series is like TheAbridgenator says, its a comedy series with lots of action. The drama for Saitaima comes from the fact that no-one takes him seriously as a superhero or even knows who he is. And most of the tension comes from watching how other heroes like Genos deal with situations that Saitaima can beat with ease but nearly kill everyone else. Plus its always fun to see how the other characters play off Saitaima and each other, the villains are always over the top hams and the heroes have interesting enough starting points of personality to never make anything boring. And to top it all off the animation…. my god the animation, those are the sorts of dynamic, fluidic motion fights people wished they could have seen in shows like dragon ball z or bleach.

@Horsecat  
It’s mostly a comedy series, and a lot of the comedy comes from all these fearsome anime villains and heroes’ reactions to a guy who doesn’t take them the least bit seriously and can end every fight with a single punch.
 
The non comedic bits are helped by other heroes fighting the bad guy when Saitama is not aware or still arriving at the scene. Saitama’s massive power is also a source of drama, as he’s quite bored by not having any challenge whatsoever, as well as having a bit of an existential crisis.

@Horsecat  
Well, it’s basically a comedy with extreme action, and Genos provides a lot of really tense fights. It’s kinda like how in Hellsing you know nobody can really beat Alucard, but he pulls it off with such style you don’t really care.

@441TheSecond  
I havent ever seen anything of one punch man, but having such an OP main charqcter doesnt break the series? I mean it gives no chance to a real conflict if it can be solved in 1 sec

@Minus  
Wasn’t even a matter of finding the little gem, he just pulverized it alongside the rest of his body with a single Serious Punch. His punches are powerful enough to SPLIT APART THE ATMOSPHERE WITH THE BLOWBACK, and it’s even implied that’s not his full Strength. Saitama is just fucking Godmode.

@TheAbridgenator  
Well just like that boss guy he beat with more than 1 punch, it was just a matter of finding that little gem and smashing it. Madoka even in her final form still needs her soul gem right? And given Saitama’s ability to hop to the moon and back, Madoka’s ranged attacks really aren’t gonna help her as much as you’d think.

@TheAbridgenator  
Whatever the issue is, he One-Punches it.

@TheAbridgenator  
One Punch. It’s over.

@Roy-of-Asturia  
Hmmmm…how WOULD Madoka vs Saitama turn out?

@Roy-of-Asturia
 
Too bad Saitama’s out of her telekinetic grasp.

Madoka, I mean Tatsumaki can eliminate her foe without physical contact.