Tomatoes, spinach, broccoli, carrots, sweet potatoes, asparagus & cucumbers are all particularly healthy. Try to aim for 10 different fruits & vegetables each day. That doesn't mean 10 servings, but even a few bites of each one is very beneficial. This is how the most healthy societies do it, such as the Okinawan's.
*I know tomato isn't a vegetable, but it acts as a vegetable in most recipes.