This is a question that comes up from time to time.
Foods with high amounts of vitamins B and C are best fresh because the vitamins are water-soluble, meaning they’re usually dissolved in food processing. In the fresh produce section, go for bell peppers, citrus fruits, cabbage, and berries.
Studies suggest frozen veggies have just as many nutrients as (if not more than) their fresh counterparts. Foods that are best frozen are those with high amounts of fat-soluble nutrients, like vitamin A, carotenoids, and vitamin E, because they’re more stable during food processing and storage (like blanching and freezing).
So when hitting the frozen aisle, go for carrots, leafy greens, and broccoli. Boiling or steaming some veggies (like broccoli and carrots) may actually boost levels of free radical-trapping antioxidants and phytochemicals like carotenoids and polyphenols.
The best bet for getting the most nutritional value out of cooked veggies is to go easy on the temperature and cooking time. Ideally cook with little to no water.
Personally my preferred cooking methods are steaming and sauté.