If this was 2012, i would have a very, very long list of people for this topic. But nowadays most of the athletes i grew up despising are either retired or irrelevant. Of course there are exceptions, like a particular rider in the Tour de France, who continues to put a stain...
But with the years, i'm not taking stuff so personally now. It's nice when favorites lose, but i'm not actively rooting against top names, like i would have a decade ago.
So, not to be a cop-out reply
Simone Biles and the whole US women's Artistic Gymnastics team - it's really irritating the over emphasizing and obsession Americans and their media get with women's gymnastics when the Summer Olympics comes. It's been this way from 2008, but has amped up in the Biles era. If she / they don't win every event it's doom and gloom. This is why, what happened in Tokyo was very satisfying for me.
Naomi Osaka
Adam Peaty
Shelly-Ann Fraser-Pryce
Mariya Stadnik
And pretty much don't want most "foreign" athletes getting medals for other countries.