My favorite was Cocos Island. Dozens of hammerheads on almost every dive, often a foot or two away, Galapagos sharks, blacktips, silky sharks, whale sharks, marlin (!), and too many white-tip reef sharks to count. But the highlight was a bait ball, featuring thousands of baitfish, now deceased, and hundreds of predators in a frenzy eating them: yellowfin tuna zipping right at you and veering off at the last instant, a school of dolphins, probably a hundred silky sharks taking turns throwing themselves through the bait and out of the water, and seabirds diving in from above.
I have also been to Bali; Belize; Bonaire; BVI; the Coral Sea; Cozumel; Derawan, Kakaban, and Sangalaki (off Borneo); Florida; the Great Barrier Reef; Jamaica; Kauai; Komodo and the surrounding islands; Layang-layang; Little Cayman; Martinique; some quarry diving in Maryland and Pennsylvania when I was in college; Maui (Molokini); some New York area wrecks when I was a teenager; Palau; Papua New Guinea; Phuket; Puerto Rico; the Similans; Sipadan; Sri Lanka; Sulawesi; Tioman; Trengganu; and the Truk Lagoon.
I would return to most of those places--and I have returned to many, but I can't recommend Jamaica, Martinique, Puerto Rico, or Sri Lanka based on the dives I did. Quarries? Hopefully not. NY wrecks? I might take another look someday.