For me, unquestionably, it is Isabeau. I've had her 3 years and these days I think she is really starting to come around. If you care to read the long version, here it is: https://www.horseforum.com/member-jou...-queen-637890/
The short version is that she was given to my neighbor when an acquaintance couldn't handle her. She had bought her from the breeder and soon discovered the mare had many surprising issues that did not show when she rode her around in a little arena for 15 minutes. Although she had a two week guarantee, the breeder refused to take her back (no kidding!) so our acquaintance gave her to my neighbor.
I thought her issues would be easily resolved with lots of quiet regular riding. They weren't. Her main problem was that when she decided she was done being ridden, she would rear up and fall over sideways with her rider. She didn't care if she was headed home or with other horses, when she was done, she expected the rider to GET OFF or she would get them off.
She would not ride out alone, haul in a trailer, face scary situations (like water or fire), or really do anything she decided she didn't want to do. She was a young horse who was just barely green broke, and to the best of my knowledge, green broke by kids.
I kept her because she was a dream to ride when she was actually riding. I just couldn't believe it would be that hard to eradicate her faults. It was. But I never gave up. I just kept quietly working with her. As BSMS says, "This will profit you. This will profit you not."
She still is incredibly fun to ride, full of pep and zip, brave as a lion, glass smooth gait, I just don't get tired riding her. I now can ride her solo anywhere, she has taken some novices safely, she is much much better riding in a trailer, though we are still not quite done with the terror of the trailer, we are very close. Best of all, we are a team now, she and I. I listen to her. She listens to me. I wouldn't sell her for a million dollars.