Hands down Roatan, Honduras 'String of Pearls'. Not only having several encounters with octopus, crabs, spotted lobster and we witnessed the String of Pearls phenomenon.
The Roatan String of Pearls occurs about an hour after sunset. It must be pitch black with no moon and lights turned off. It is the mating display of ostracods which are tiny organisms about the size of a tomato seed. They act much like a firefly emitting a little light in a string sequence to attract a mate. Based on the spacing between the lights the female attaches herself to a corresponding suitor.
During your lights out move your hands around and watch the bioluminescence react to the movement of your hands. While messing with this your eyes will adjust and if you are lucky the string of pearls will leave you in awe. I could spend an entire dive just watching it occur.