My mother. Hands down. Serious mental issues (untreated, mind-boggling denial). How? I'd hardly know where to start. I'm hard-pressed to think of much that hasn't been negatively impacted. She never admitted, let alone apologized. Ever. For anything. I can recall only one compliment growing up.

It was what it was. (And it was a long time ago.)

Despite this, I was doing quite well before the BP. That first severe depression came at 20. It side-swiped me out of nowhere. I was having a GREAT time for months on end before that. Getting no help at the time (obviously), these chunks of inexplicable-ness continued another couple of decades before I learned it was bipolar.
To my dad that he is NOT the person my mother made him out to be all those years. It may have taken 36 years in the dark, but it was good to find out. And that he didn't reject me when I did contact him.

To my mother? Nothing comes to mind.