Not considering aliens as monsters (because those soldiers from Boros could be thought as monsters but not counting them) and leaving Garou "the half monster" aside, who is the strongest monster in One Punch Man?
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For me, there is no way to gauge a specific monster as the strongest, unless the mangaka explicitly states that they are strong or if a chapter would be released detailing the monsters' exact prowess.
However, included among the top of the monsters are Boros and Orochi, both Dragon+-level monsters (but not stated as God), and Golden Sperm, who Murata says he is not sure if Orochi or him (Golden Sperm) is stronger.
As of now this https://onepunchman.fandom.com/wiki/Orochi Orochi is the strongest monster in the series because hes the leader of monster association means that he has Dragon+ level threat