The fresher you buy, the better. Nutrient-wise, you'll be getting more bang for your (calorie) buck when you stick to fresh fruits, veggies, whole grains, and legumes. Don't write off frozen produce either. Frozen just after harvest, the nutrients are locked in and freezing provides a longer shelf life to these fruits and veggies.
Choose lean organic, grass-fed meats and organic dairy products since these items are made without the use of hormones.