To chose one would be impossible. Plus there are so many that I have never had the opportunity to try. Even when I was in New Zealand for work, I never had the chance to try fresh green lipped mussels.
Most of the fish that I prefer are ones that I or my wife have shot. Freshness is never a question then!
I have never had spiny lobster that one of us has not caught ourselves. However, my recipe for Lobster Diablo requires that I use defrosted tails as it is impossible to remove the raw tail meat when it is right out of the ocean.
I have never met a crab that I dislike, but until recently have always prefered blues. Here in Florida, in the past few years, a deep water species have been harvested commercially called Golden Crabs. These are a large spider like crab, smaller than kings but larger than opies (queen). Very sweet and much easier to pick than blues. Of coarse, we are very lucky to have stone crabs here as well.
Conch is very high on my list, but it is unfortunate that they are still off limits to take here. Our wonderful gov imposed a 10 year moratorium on them back in the '70s that has never been recinded, even though the numbers have rebounded enough to rival our 1.7 million alligator population. Gators can now be taken under strict limitations, but no conch.