For aficionados, nothing says summer like just-picked tomatoes. They're at their peak, and chefs are celebrating with menus designed to showcase the scarlet globes. Alain Ducasse at the Essex House in New York City serves an appetizer of roasted tomatoes in a tomato gelee topped by tomato tuilles. Melisse in Santa Monica, Calif., offers an eight-course tomato tasting menu, and several eateries bake yellow-tomato cupcakes. The heirloom tomatoes that chefs like, including purple brandywines, green striped zebras and fuzzy blues, are popping up at farmers' markets and gourmet food shops, and the seeds are big sell- ers. But whatever types they buy, chefs say they look for the same qualities in tomatoes. They should be vine ripened, fragrant and deeply colored: the reds rosy, the purples uniformly purply. Shape doesn't matter old-style bumpy tomatoes are often tastiest but they should be heavy for their size and give ever so slightly when gently squeezed. Store all tomatoes at room temperature; refrigeration makes the flesh mealy and kills the flavor.