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Certainly Sklansky has to be fairly high up on the list. He's a pioneer thinker in terms of many facets of poker, and he has managed to write several books that were accessible. I've gotten the impression he does a fair bit of computer simulation and calculation to verify different scenarios as well. Ferguson is probably doing the same and is probably on the list.

That said, "intelligence" is a fluid concept. You can have someone who scores a 130 on their Stanford-Binet, and another who scores 160, and still have the 130 with an advantage in all sorts of purely intellectual pursuits, because some people excel at different sorts of things - inductive reasoning, deductive reasoning, spatial visualization and manipulation, language skills, and so forth. Personally, I'd think that inductive reasoning, pattern recognition, and deductive reasoning were skills most critical in terms of thinking to poker success, grain of salt and all.