For me, it was having my firstborn.
I'd had a difficult and painful pregnancy, made much worse by me literally having a phobia of pregnancy and childbirth (not just a bit of fear, proper full blown tokophobia).
After 9 months of panic attacks, dark thoughts and crying myself to sleep often, I voluntarily walked down to the operating theatre, shaking, to have a section (natural birth was out of the question for me) and half an hour, I met my beautiful daughter.
All the love, and the hormones combined with the sheer relief and pride in myself that I managed it (with a smile on my face in the end) meant I felt like superwoman. It took me many months to come down from that high.
I've had many amazing moments in life (and the birth of my second was also amazing, but less overwhelmingly so because I knew I could do it, and was expecting to feel amazing) but the combination of fear, effort, love etc, I wish I could have bottled up.