I have two favorites:
1.) Relatively early in my dive days, a buddy and I ended up improvising our own dive plan in a shallow area with three wrecks. I took on the role of navigator, and despite my anxiety about potentially getting us both lost, I managed to navigate to all three wrecks and back to our boat within in our time frame, using only the position of the wrecks, the reef, a slight current, and the sun overhead. It was really empowering, to know I could navigate successfully!
2.) My favorite animal-related dive was this past February in Turks & Caicos, when our group encountered a humpback whale and her calf... what an incredible experience!