I can't name a favorite because I have had so many good experiences that all seem about equal but in different ways. I have been to more places than some divers, though, so perhaps someone may benefit from my thoughts on a place they may be considering visiting. So, here are my highlights in alphabetical order:
Akumal area in Mexico: The reef diving is pretty good, and the Cenotes (caves) are wonderful.
Aruba: The diving is generally mediocre. It has a good wreck (Antilla), but the operator I used to get to it (Pelican) led a regimented group with no opportunity to explore. Dive Aruba with Clive Paula gives you the best bottom times ever, but you should be a pretty confident and competent diver to go with him--there is no handholding.
Australia (Great Barrier Reef):How much can I claim to know having only a dozen dives between Lizard Island and Cairns? Still, I can say that this should be on anyone's list of places to go. Be sure to take a liveaboard so that you can get plenty of dives out in the ribbon reefs area rather than the patch reefs closer to shore.
Belize: Ambergris Caye has pretty good reefs with deep spur and grove formations. Opinions differ on the Blue Hole, but the dives off of Lighthouse Atoll and Turneff Atoll are great.
Bonaire: Maybe it was because I was expecting so much, but I felt that, good as it is, it is a bit overrated. The island has a wealth of great dives, but they are somewhat uniform in structure, so you get very used to sloping walls with brown-green star coral. This sounds like a negative review only in comparison to the raves others have given it--it is a great dive location.
Chuuk (Truk): Trip of a lifetime if you like wrecks, and I do.
Cayman Islands: Cayman Brac has very nice dives in a secluded setting with only one real dive resort. Little Cayman is even more secluded, and Bloody Bay Wall is one of the best walls you will ever dive. On Grand Cayman I dived mostly the east end, which is less popular. It has great coral formations.
Curaçao: This is one of the most underrated dive locations in the world. Its coral formations look very much like its neighbor, Bonaire. The island itself is much more interesting.
Cozumel: Always a great site for the beauty of its coral formations, espcially if you like swim throughs.
Fiji: Wonderful coral formations and fish life. Brilliantly colored soft corals.
Florida: The east coast north of Fort Laudrdale has some interesting wrecks and low reefs for drift diving. The Key Largo area has decent shallow reefs, and the Spiegel Grove is a great wreck.
Galápagos Islands: Another trip of a lifetime. After a thousand hammerheads or so, you actually can get a little tired of seeing them. Whale sharks are hit or miss. Lots of unique species. You get the idea when the fish book for the area has the word "Galápagos" as a part of the species name for so many of the fish.
Maui: Reasonably nice dives on a wonderful island. The lava tubes are beautiful.
Puerto Vallerta: I got my OW certification there, but it would not be a place I would otherwise seek out for diving.
Saba: I only did one day (3 tanks) there while visiting nearby St. Maarten, but they were three great dives. If you are staying on St. Maarten, be sure to fly there (rather than take the rough ride on the water taxi) so you get all three dives. The first one (deepest) is the best, and it is the one you will otherwise miss. UNderwater volcanic pinnacles with pristine coral and an abundance of fish that really don't care that you are swimming with them. Make sure you get a taxi ride with a native to show you around.
St. Maarten: Only average diving, but it's a fine island to visit otherwise. Good base for a hop over to Saba.
San Carlos (Mexico): Pretty nice diving with sea lions, but not a lot more than that. It has a limited range of options.
Thailand: Great fish and good coral, but you can't believe how much damage has been done by dynamite fishing. I was there soon after the tsunami, and the damage from that was also evident.
Yap: Beautiful walls and lots of opportunities to watch Manta Rays. The islands themselves are very beautiful and interesting.
