A few off the top of my head, the first 3 were all in the caribbean.
1.) We were moving across a fairly flat area exploring some new deep dive sites, and a great hammerhead appeared out of nowhere, about a 9-10 footer. He spooked quickly and ran immediately after my dive buddy pointed at him.
2.) My friend snapped a picture of me coming up over the side of a massive wreck just as an eagle ray was passing overhead. I actually use that picture on my website.
3.) Some shark diving we did on some Atlantic side wrecks mainly drawing in reef and nurse sharks, but still really cool with several of them circling the wrecks.
4.) Diving on a WWII Helldiver just off Jupiter FL. I thought it was really cool to be diving on an airplane like that, but the dive in general was quite challenging in itself. Strong current, some of the divers didn't make the site, long drift decompression, etc.