My experiences are that southern stingrays are relatively common, followed by nurse sharks (a few per week) and eagle rays (ditto). Probably one reef shark sighting every ~25 dives or so, usually off the wall (deep). Then there's what I'd call "known residents", such as a Lemon that was known to fairly reliably hang out at a particular spot over in Little Cayman's shallows, etc. From there, I've had IIRC three manta sightings and one hammerhead out of probably ~700 dives. In general, I suspect that some of this includes some bad luck from the perspective of 'time of year & day' factors, since a friend who has done a lot of shore dives in the Bloody Bay region reported pretty frequent reef shark sightings in the late afternoon, after most of the dayboats have long since gone home and the reef has quieted down.
-hh