High intakes of fruits and vegetables are known to be protective of disease, although the exact reason for this is not known. However, it may relate to the phytonutrients contained within plant foods. Most people in developed nations consume a typical Western diet that is low in whole plant foods, including fruits and vegetables, and high in animals products and processed and refined foods. Such a Western style of eating is now considered a major driver of lifestyle disease such as cancer, cardiovascular disease and type 2 diabetes. Eating a traditional diet that contains more plant foods is protective of disease, and some people may be interested in abandoning the Western diet to adopt a diet based on more traditional eating patterns with higher intakes of fruits and vegetables. Faced with this change, many are unsure as to which fruits or vegetables to eat and how to prepare them.
In answer to this I can only say that the best strategy is to eat a large variety of different fruits and vegetables. The greater the number of different kinds of plant foods the better. Try to incorporate as many colours into your diet as possible from plant foods, as this will increase the range of phytonutrients absorbed and widen the protective effect of the diet against different diseases. As for preparation, it really doesn't matter how they are prepared as long as it is to taste and they remain close to their whole state. Juicing fruit and removing the fibre for example is no better than consuming soft drinks according to studies. Some evidence suggests that certain preparation methods for vegetables are better than others for retaining nutrients. However, the reality is that while steaming might produce slightly better nutritional profiles than boiling, eating any plant foods is a big improvement over a Western diet and if the intake is high enough even sub-optimal preparation methods will provide some benefits.