Who is the strongest monster in One Punch Man?
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.