Let's be real here. Goku could beat him. Naruto could beat him. Fucking Luffy can beat him
He's a joke character in a gag manga, nothing more. He's not meant to be taken seriously, so stop with the power level shit.
Except his ability is dependent on actually punching somebody. Most of the time they don't know who he is so they underestimate him. The easiest way to defeat Saitama is to just not let him touch you.
You eventually get bored to call on the same faggots, also shit's gets even more ridiculous when DBS babies come in. Seriously they should just kill themselves already of /a/.