Bald Bull. He's not just the biggest asshole in Punch-Out, he's one of the most assholish video game villains ever... and possibly one of the biggest assholes in the world. Between his perpetual look of disdain... the fact that he's approximately four times bigger than Little Mac... and the way he busts out his bull charge knockout move just enough times that he's definitely going to knock you down at least once... he's evil.
Soda Popinski. He fights you drunk and openly laughs at you before you start boxing and when he knocks you out. He also dodges most star punches, has this one jab that's always infuriating... and, for some reason, he's purple. Honest to God, purple.
Don Flamenco. He's not that hard to beat, but he's so smug and creepy. He's too realistic.
Von Kaiser. Just because he's really easy to beat doesn't make him less of an asshole. Maybe it's my built-in prejudice against Germans. I don't know.
Super Macho Man. Same deal. They put a Southern California meathead in the game. He'd be higher on the list, except he's a little vulnerable thanks to the fact that he's got a receding hairline and he's clearly just a sad middle-aged man clinging to his "glory" years. (Which, by the way, is a lot of nuance for an 8-bit video game character. Well done, Nintendo.) His spin punch feels assholish, but not terribly. And his taunts, like "I work on my tan harder than I'll have to work on you," just feel like he's mailing them in a little.
Great Tiger. He's a taunter. He steps away, where he's out of reach, then taunts you. And you know he knows Little Mac can't move that way. So obnoxious.
Doc. Possibly the worst boxing trainer of all time. He rides a bike while you run... he only rubs your shoulders hard once a match, and you have to hold Select to make him do it... and he literally offers the worst and most generic advice in boxing history.
Piston Honda.He's like an aspiring asshole, but he doesn't quite get there. He mostly seems obsessed with giving shout-outs back to Japan. (By the way, how self-loathing are the Japanese programmers at Nintendo who created this guy? One of his taunts is "Sushi, Kamikaze, Fujiyama, Nipponichi." Were they trying to play to Americans, saying, "Yep, our language sounds funny and even our national heroes just spit out nonsense words"?)
Mr. Sandman. He's here because he's good, and he can be frustrating. But it's hard to have any animosity toward him. He fights straight-up... his rapid uppercut move is powerful but reasonably defendable... and his taunts are basically just generic ("I think you're gonna have a nightmare tonight.")
Mike Tyson. In real life, maybe. In the game, not at all. He's just really, really good. The satisfaction you get when you finally practice enough to beat him comes entirely from the challenge. (Unlike, say, beating second Bald Bull, when you invariably stick two middle fingers up at the screen and laugh at his TKOed corpse.)
Mario. I couldn't put Glass Joe here, he's like a guy who's been forced to box because he somehow got under the mob's thumb and still needs to pay them off to save his family. And I couldn't put King Hippo here, because he's really cuddly and sweet and, if he ever lasts to round two, he invites you to go to lunch after the fight. (Even though listing his weight as "???" is borderline asshole behavior.) So I'm going with Mario, who refs the matches... and sometimes arbitrarily slows down or speeds up his ten count.