This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Hajime no Ippo
Hawk's red eyes... just... his red eyes.
In all seriousness, Bryan himself is perhaps one of the scariest characters, due to his animalistic insanity.
Bryan as a whole is terrifying for multiple reasons- his style is anything but orthodox. It's not even boxing. It can really only be described as the kind of fighting a man would engage in during a life-or-death struggle,swaying around to avoid being hit and flailing his fists out at every angle, using as much power as possible behind his swings. In a way it can be compared to a rabid chimpanzee attack, completely random and off-kilter where there's really no way to defend yourself but to run; and while the style does have its weaknesses, they aren't critical enough that they can be exploited. And he does all this with an insane grin, sometimes a sadistic laugh... and those goddamn horrifying red eyes. That's not the worst part about it, though. What's really terrifying is that when he's really into it, beating his opponent into a pulp, he gets sexually aroused from it, sporting a visible erection under his trunks. During his bout with Takamura, he even says he's going to cum from it. Everything about his boxing and his attitude is demented and fucked up.
And another one in chapter 912 is a bit more disturbing, coming from Itagaki of all people…
While not outright terrifying, Ippo's one shot K.O against Kojima is chill-inducing. He takes the brutal counter, and you think he's going to go down...but he doesn't. And he comes back with a vengeance. A bone-shattering, rage-induced, career ending vengeance.