What is the healthiest vegetable or fruit to eat?
Some fruits and vegetables are considered super foods (ie. kale, blueberries), but the healthiest diet is one rich in a wide variety of fruits and vegetables, and not one that simply focuses on those that are antioxidant rich, or fiber rich, etc.
Check out this list to find out why each is considered one of the world's healthiest foods: http://www.whfoods.com/foodstoc.php
After softpunk's excellent answer, I can add advice I was given -- "Eat your colors." Orange, red, green, blue, even some white, carry great nutrients. Eat whole food nutrition, make your plate sing, and you'll have a healthy diet.
Don't forget nuts, grains and good fats!