Remove husks from corn and pull off all silk. In a large saucepan of boiling water, blanch ears of corn just until barely tender, about 2 minutes. Drain ears and run under cold water; let cool completely.
Place chopped tomatoes on paper toweling to absorb some of the liquid. After 5 minutes, press top with another sheet of paper toweling, and place tomatoes in medium-size glass or ceramic bowl. Cut kernels from corn cobs; add to bowl. Stir in red onion, basil leaves, white-wine vinegar, extra-virgin olive oil, 1/2 teaspoon garlic salt and pepper. Let stand at room temperature to mellow flavors, about 30 minutes, stirring occasionally.
Meanwhile, cut tops from tomatoes. Scoop out and discard flesh and seeds, leaving hollow shell. Turn upside down on paper toweling to drain for 5 minutes. Sprinkle with garlic salt. Fill each tomato shell with 1/2 cup tomato-corn salad. Serve or chill until serving. Makes 8 servings.