I'm really into snorkeling photography and I spend a lot of time on flickr looking at uw photos. Based on the photos, the two places that always stand out to me are first, the Maldives, followed by the Red Sea. Fantastic uw viz, diversity, corals, and fish. I have not been to either location, however, although I would dearly like to go. I have been, numerous times, to the Caribbean, Hawaii, and the Sea of Cortez off Baja California Mexico. In the Caribbean, I would probably rate Belize the best for snorkeling, although for the good stuff you have to go by boat or stay on one of the tiny cays on the offshore "atolls", which is a disadvantage. Utila and Roatan have good very snorkeling right off the beach, and Grand Cayman is pretty good too, although I wouldn't rate it quite as high as Roatan. Hawaii has many exotic reef fish because it is part of the Indo-Pacific faunal area, but I prefer the Caribbean for snorkeling. Maybe it was because I always went to Hawaii in the winter when the sea condition tends to be rougher, but my memories of Hawaii are snorkeling in small protected coves and frequently having to deal with surge. You didn't the opportunity for "endless" exploration like you do in the calmer Caribbean. The reefs in the Caribbean are also much prettier with branching coral, soft corals, and sponges. The Sea of Cortez is interesting in terms of biology because the eastern Pacific is its own unique faunal region, but most would rate it lower for snorkeling than the other locations I have mentioned.
I have responded previously to posts like this by saying I have yet to find my "favorite" snorkeling locale. I have enjoyed all the places I have been, but no one place stands out as the best. Each place has its advantages and disadvantages, and the topside environment is also part of the experience.
