For me, the first to come to mind are Horghall and Inari.
I never use Horghall. Ever. Without pausing to think, I can name 20+ better tanks, and defensive tank is the only role he’s suited for in the game. His tile damage is a joke, he’s too slow to fire, and when he does it’s too late to be helpful and more like throwing weeds at someone than it is like calling upon nature to hold them down. He’s awful, and as great as his artwork is, I’ve hated that I ascended him from nearly day one.
Inari is deceptively enticing to ascend because she seems on paper like she might be good. She’s not. She’s too slow to be viable, and even when you can get her off, it’s a coin flip as to whether or not she’ll be useful. Almost every time I take her into a fight and charge her special, I’m wishing in that moment that she could kill something. But she can’t. She’s just going to sit there and maybe do a little dance that’ll prolong everyone’s death. Maybe. But probably not.
Anzgogh is another I can’t stand. He’s another that seems on paper like he might be alright, but the longer into a fight you go with him, the more useless he becomes. And since he charges so €*#@%< slowly, he’s going to feel continually subpar. You’ll be happier to fire virtually every other fire hero’s special than you will be watching Anz’s cool animation do not a whole lot.
Lower on my list of regrets, having played the game now daily for years: Kestrel, Richard, Margaret, Justice, and Quintus. There are others who are bad that I have but haven’t ascended, and more who I don’t own and wouldn’t dream of pulling the trigger on, but I won’t speak to them, as I don’t have firsthand experience of what a terrible, awful choice I made.