A.W.
Most difficult - when my DD's biological dad terminated his parental rights to her. It was bitter-sweet. The other most difficult thing, for me, was changing my parenting style. I learned the hard way that my DD needed me to let go a little and be more of a mentor parent instead of a smothering mother. My preferred parenting style is attachment parenting, so as she got older, she was less interested in my parenting style.
The best experience - when my oldest DD actually thanked me for homeschooling her. She said that she enjoys learning now and has a better social life than when she was in public school.