The seafood at this busy downtown street stand, known to fish-loving insiders, is exquisitely fresh, set on ice and behind glass. The ceviche of crab, fish, squid or octopus, made with chopped tomato, chili, onion and cilantro, augmented with lime juice and olive oil, is as good as it gets. A crisp, recently fried corn tostada topped with fish and a few slices of avocado is the best way to sample it.