Jo Robinson, http://www.eatwild.com/jo.html, says the storage and preparation of vegetables governs how many antioxidants and other nutrients you get.
Veggies today are not the same as the veggies our ancestors grew and ate. They are less nutritious, less fibrous, and more pulpy than 400 generations ago (the good OLD days).
Generally speaking--the more bitter the item, the more antioxidants. But we don't like bitterness these days.
SHAPE. Lettuces with broad leaves and more exposure to sun are usually healthier. Iceburg receives less sun exposure.
SIZE. Smaller tomatoes have more lycopene.
STORAGE. Store veggies in plastic gad with small holes punched in.
COOKING. Some veggies--carrots for example--have more nutrients cooked. Other veggies are better raw.
CHOPPING. If you chop garlic, let it sit 10 minutes before heating.
Eat all veggies 2-3 days at most after buying them.