Excerpts from a long - and annoying - list...
A dozen Scalloped Hammerheads while I was waiting to be 'rescued' as a victim for a rescue course. (Managed to find the same school the next day, though, with video camera in hand).
More Thresher sharks and Giant Mantas than I care to think of when I was dive guiding at Malapascua.
Schools of feeding Grey reef sharks, once while doing Open Water dives with students, once just after I'd successfully demonstrated that the housing manufacturer's depth limit was correct.
Sixty or so Spotted Eagle Rays dancing, again just after a 'whoops' camera moment.
The only time I've ever seen an adult Tiger shark underwater.
My all-time favourite, three baby frogfish hanging upside-down from a ledge, arranged in size order and all the same shade of yellow. The best bit is, I had my camera... I'd set up in a hurry ('Quick! The boat's leaving for an awesome macro site RIGHT NOW!') and left the lens-cap on inside the housing...
I probably shouldn't be allowed to play with cameras.