Shawn Michaels would have a say in that argument.
I remember back in the pre-Attitude era when HBK was champion and fighting everyone and anyone. He was receiving pops from the ladies because he made their panties wet, but the guys couldn't stand him and seen him as a little bitch who didn't deserve the WWE title.
The only difference is the fan base. .
You make a good point about the HeartBreak Kid. But I think Cena's big claim to most-hated-face is longevity. Cena has been a face-at-the-top for about 10 years. That's a long time. And he's been getting a mixed reaction for MOST of that time. There were other face champions with decade-long runs at the top, but I don't think they were getting mixed reactions--fans were maybe getting bored, but I don't think they were getting boos.
Cena has been getting booed as a top face for at least five years, maybe longer. Can anyone else in wrestling history say that?