My kids learning to ride a bike - the moment you release your hand for the first time and they just go and go. When my son learned checkers, and then when he beat me the first time. When my daughter told her first original joke at a family dinner and everyone died laughing.
The moments truly never stop. Every single day they amaze and surprise you, fill you with so much love and joy and appreciation.
One time Bill Gates was asked what gave him joy and without missing a beat he said his children. Nothing is greater, nothing gives you more meaning, nothing is more ultimate than the sacrifice and patience and wonder and fulfillment of having children.