Ok, I'm biased, but...Galapagos. Whale sharks are seasonal, however, when I went to snorkel with the humpbacks in Silver Bank, my first thought was that our whale sharks are larger. Almost 99% are pregnant females. Huge schools of Hammerheads, Galapagos Sharks, Silky sharks, Black Tip sharks, White Tipped reef sharks, Mantas, Eagle Rays, Stingrays, Cow Rays, Dolphins, tons of turtles, Mola Molas (sunfish), massive schools of jacks & barracudas, tropical reef fish, puffer fish, trumpet fish, octopus, lobster, nudibranchs, sea horses, sea lions, fur seals, marine iguanas, penguins, sea stars, moray eels, garden eels, snake eels, blue footed boobies, flightless cormorants, Orcas and humpback whales...The diversity and quantity of marine life is why the Galapagos are on every diver's bucket list and why no one who goes ever regrets the cost, just that it spoils diving elsewhere for them. Plus the land before time vistas when you're anchored with 4 active volcanoes in view.
And sorry Doc, land based diving in the central islands simply does not compare to diving the northern and westerns sites from a liveaboard. It's cheaper, yes, but not comparable. The analogy I use is that if you walk downtown in Marfa, TX, you're bound to see people, but not nearly as many as the numbers and diversity you see walking around midtown Manhattan. And there is no manta season. Manta sightings are more site based than seasonally based.
Go in high season when the whale sharks are around. Typically they begin showing up in June and are there until mid Nov. Scientists tracking whale sharks have shown individuals are always different and only spend a day or two in the north before moving on. It's a migration route that they assume has to do with calving, probably in very deep ocean northwest of the Galapagos, though no one has ever witnessed whale sharks mating or calving.
Galapagos is like the perfect storm of currents: The warm Panama current from the north east, equatorial currents from the west and the polar Humboldt current that arrives in June. High season does concur with reduced visibility, but it's the nutrients the currents bring that causes the explosion of life. I personally don't care if there's 100' visibility if I'm watching hundreds of hammerheads 25' in front of me, like I'm on the side of their highway and they are the traffic.
Thanks!