I have a lot of happy memories, but I think my happiest memory was being present for my youngest sister's birth. I was fourteen when she was born and wasn't supposed to be in the birthing room, but we had a storm and my uncle couldn't come to pick my younger sister (9) and I up so we were present for her birth. It is weird being 28 now and my youngest sister being 14, the age I was when she was born. We aren't as close as I would like to be, but I loved spending the first five years of her life with her and getting to experience that.
I likely won't have children myself, so it was very nice to be around to watch her learn how to walk and talk, to get her first ouchie, to teach her how to ride a bike with training wheels, etc.