I had to vote Windy, she was the only rainbow pony I had as a child and I gave her away. I had brought some ponies to my Grandma's to play when we went to visit one day, a cousin of mine who I never really knew happened to be there that day too. We played with my ponies all day and had a great time, she said she didn't have any ponies and played with Windy the whole time. At the end of the day when we went to leave, I told her she could have her. My parents were very shocked by this because I loved my ponies as much then as now and giving one away was just unheard of but I liked this cousin and I guess I was feeling nice. My mom later told me that was a very sweet thing I did because this cousin didn't come from the best home life and her parents struggled with money.
Windy was the only pony from my childhood I ever gave away, I still to this day have the rest of my ponies from then. However, as an adult buying ponies I have probably gotten Windy a dozen times, she's been duplicated more than any other pony in my collection. She always seems to end up in lots I get or at a flea market and each time I get one I think back to that story and remind myself, everything we give comes back to us.