I'm going to narrow this down to a completely specific moment.
This was Eloise and me on my wedding day. Eloise had just turned two. In the middle of the ceremony she came up to me and asked to nurse. It was a sweet, beautiful little moment, and luckily she was okay with not doing it right then and there, and she wrapped herself up in my dress and hung out for a bit as we proceeded through the ceremony.
This is perfect for a "happiest moment" because for one brief shimmering point in time, everything was right in my world.