Vegetables are best fresh or fresh-frozen. Current practice in developed countries is to flash-freeze vegetables shortly after harvesting. The vitality of fresh-frozen vegetables is preserved for a while, but only for weeks, not months.
Eating raw vegetables is a positive goodness, though at some sacrifice of accessing nutrients during digestion.
Steaming vegetables makes the vitamin content more readily digestible by breaking down the cellulose, while leaving the structure, and most of the vitamins, intact. Cooking vegetables in water dilutes the vitamin content, and so is not recommended, unless in a stir-fry situation, where the entire contents are consumed.