It depends on what you are looking for and who you are going with. If I were going with a non-diver or had to have nightlife in order to enjoy a dive trip, I would pick Grand Cayman, as there are plenty of topside activities and nightlife available there, and not on the other two. There is not a big difference between Little Cayman and Cayman Brac, other than a few more topside sights to see on Cayman Brac and better wall diving on Little Cayman. LC is also a little quieter and more secluded, but Brac is quieter and more secluded than about 90% of the other locations in the Caribbean. I am going to Brac next month, but I have been to both Brac and LC. The reason I chose Brac was purely economic. If money were no object, I would be going to LC instead. Either way, I rank the diving on Brac (twice) and LC (once) ahead of what we have seen diving Cozumel (twice), Belize, and Roatan (twice).