From RealAge
I think you should always be choosing the healthiest vegetables to eat (getting the biggest bang for your buck), because when in doubt, we really should eat more vegetables!

If you're a broccoli lover, you've picked a winner. From a list of 10 of the most commonly consumed vegetables in the country, broccoli holds the top spot for having the most phytochemicals -- compounds everyone's urged to consume because they protect against chronic diseases. At the bottom of the list? Cucumbers, described by some people as "water you can eat."

1. Broccoli
2. Spinach
3. Yellow Onions
4. Red Peppers
5. Carrots
6. Cabbage
7. Potatoes
8. Lettuce
9. Celery
10. Cucumber

Red peppers actually beat out broccoli in terms of having the highest levels of antioxidants, so I'm not sure how come it ranks only #4.  Especially when they are also high in vitamin C.  Please don't take this as a definitive list.  This list comes from the most commonly consumed vegetables.  I would love to see the most commonly consumed vegetables change to be the ones with the most nutrients.  Lettuce, celery and cucumber aren't very nutritous - would you agree?

The same researchers also have rated the most popular fruits.

1. Cranberries -- with the most phytochemicals and the most antioxidants -- topped that list, followed by
2. apples
3. red grapes
4. strawberries
5. peaches
6. lemons
7. pears
8. bananas
9. oranges
10. grapefruit
11. pineapple

Food scientists aren't ranking veggies just to satisfy their curiosity.  Maybe one day the rankings could help us choose the healthiest vegetables and fruit over the not as healthy one. 

References: Antioxidant and antiproliferative activities of common vegetables. Chu, Y. F., Sun, J., Wu, X., Liu, R. H., Journal of Agriculture and Food Chemistry 2002 Nov 6;50(23):6910-6916.

Antioxidant and antiproliferative activities of common fruits. Sun, J., Chu, Y. F., Wu, X., Liu, R. H., Journal of Agriculture and Food Chemistry 2002 Dec 4;50(25):7449-7454.