There are over 500 plants in the genus allium, but the antioxidant superstars are garlic, onions, shallots, and leeks. These vegetables contain flavonoids, vitamin C, selenium, and sulfur compounds that have been shown to have potent cancer fighting properties. They help the cells dispose of carcinogens and prevent heart attacks and strokes by lowering cholesterol and blood pressure. They also prevent blood clots and may play a role in the prevention of allergies and asthma. Even when cooked, they seem to have antioxidant capabilities.