1) A dead critter, sadly, but there's a marlin skeleton in the Turtle Cave at Sipadan Island, Malaysia. It's in a chamber about 200' in the cave, up on a natural shelf. I have heard it was placed there by someone, but don't know if that's true. Either way, it's still a very cool thing that not may people have seen.
2) During a dive at Mabul Island we were taking photos of a couple frogfish sitting side by side on some sort of wood structure that resembles a small building frame or something. Anyway, I notice a shadow moving right over my head, and I look up to see a very large turtle coming in for a "landing" right on top of where the frogfish were. The turtle ruined that photo op! Diving in Malaysia, you will see so many turtles that they begin to lose some of their appeal after a while.
3) Last summer during the Cozumel Invasion a good sized batfish was evidently annoyed by us on a night dive. He slams into my girlfriend's side then turns and comes straight into my mask then disappeared into the dark.
4) Gulf of Mexico last summer. We were at 184' directly under an oil platform searching the bottom for dropped goodies. The waters were nice and clear until about 145' then they got very silty. I was looking to my right when my buddy taps my shoulder and points directly in front of us. I look up to see a very large fish moving slowly. So large that in the very silty water I couldn't see the whole fish! We both froze, hovering above the bottom, hoping that it left us alone. As we made our way up to do our deco stops I was (and still do) wondering just WTF that was. My best guess is a grouper...or maybe a dragon.