Personally I like the ceriths, but they're not as hardy as the astraeas. And I like the margaritas: they do good glasswork, and can right themselves when they fall over--astraeas can't.
But for general use: the little strombus grazers, sometimes known as colonistas, and the stomatellas. They get into nooks and take care of their own reproduction. They're hitchers. I don't think you can buy one.