Iceberg lettuce, cucumber and celery do not cut it. Large-scale commercial growers tend to emphasize hardiness over flavor and texture in developing many varieties of vegetables. Vegetables displayed in super markets often appear uniform, sturdy and fresh, but probably don't taste as good as locally-grown produce. However, remember that most vegetables sold in supermarkets have adequate flavor and nutritional quality, and probably will be very good if properly shipped and handled. Shop frequently, select the freshest produce available, and use as soon as possible since vitamins degrade after 4-5 days.
Color is a good clue to nutrient content.
Choose deep green, yellow and orange vegetables since they contain the highest amount of beta-carotene and other nutrients. In general, frozen vegetables are preferable to canned. If the vegetables are flash-frozen soon after harvest, they retain most of their nutrients. Though they contain enough nutrients to be worth eating if nothing else is available, most canned vegetables lack flavor and texture of fresh or frozen. Save money buy produce in season when possible.