Try all of them. My own experimentation showed, however, that clams are far more difficult to open as I had to use a hammer, and the clams were always too small to be worth the bother of getting at the flesh inside. Raw mussels are a cheap alternative, but I have noticed that if I overindulge in them in the long-term, that they pass through my body rather quickly, implying that they are not being easily absorbed for some reason. Raw oysters I have never had a problem with. Raw mussels can be an acquired, very bitter taste at first, and you need a metal walnut-cracker to break up the shell so that you can pick the shell-parts off and eat the flesh inside. Raw oysters just need oyster-knives, of course.