Deep Sea King by far. It's probably unfair because he got an entire arc for himself, and while he is kinda fodder now, he really did make a big impact when he first came out and even more so than many other monsters.
Throughout the entire story, he's really drawn menacingly to amplify the fact that he is a monster, with a huge hulking physique, having some kind of creepy smile or laugh like a horror movie character. He's often drawn with lots of shading and in shadows often His design is great. I like how there's a human-ish aspect to his design, like the clown face and the human features among the monstrous fish body, to show how much of an abomination he is.
As for story, via being one of the only 2 monsters who has an arc dedicated to themselves, it's no surprising he has a lot of time to flesh himself out. In his first outing, he casually defeats 2 A Class heroes and eventually claps an S Class, all while casually mocking and toing with them, especially with lightning max. He then nearly kills sonic and forces him to retreat. the story further tries to hype him up by having Saitama put on a serious face when he hears an S Class is down (Although we know how it all went down). He proceeds to tear Genos' arm and then creepily chase after the humans, and even uses a child as distraction to beat Genos.
Granted, a lot of this is child's play now, but this was during OPM's childhood, and until then, we didn't have much info on the A class and S class other than the fact they are very powerful heroes.