I don't think it's that cut and dried. GC has some spectacular wall diving on the north and east. Babylon, for instance, is a breathtaking location as is Ghost Mountain and many others. There is nothing like these sites on Bonaire where all of the diving on the west and north is either reef or mini wall. If you want lots of sponges, soft corals, walls and a chance to see pelagics, then GC tops Bonaire in my opinion. GC is pricier than Bonaire, but in the off season, you can find some bargain packages at places like Cobalt Coast and Reef Resort and dive the north or east on a budget. Not all of GC is built up either, once you get out of Georgetown and Seven Mile Beach, the rest of the island still has some Caribbean charm. Bonaire has great easy access shore diving which is great for macro but no real walls.
Little Cayman has both of these beat by a mile when it comes to the diving, but as pointed out, there is not much else to do and it's an expensive package, even in summer. We alternate between all three islands and like them all different. If I had to do a forced rank order it would be: LC > Bonaire > GC. I don't think you can go wrong with any of them, it's just a matter of what you want and can afford.
As far as "prissy dive rules", I disagree with the comment from Doc; I've had far more aggravation on Bonaire from martinet dive masters than on GC or LC.