I especially like frozen chopped spinach. The kind that is all in chunks and you can pour, not the huge brick of ice. I microwave the spinach for a few minutes, squeeze out the water and then add it to pizza, sauces, wheat berry type salads, pasta dishes, under fried eggs - whatever. It's always good with feta. It will make whatever you are eating twice the size and only add heft, nutrients and fiber, no calories. So you get to eat a nice mound of food, not just a small scoop, so to speak.
I also like sweet corn and peas, which I don't count nearly as accurately as I should, and frozen baby lima or soy beans. These can be thrown into or on anything for added protein, fiber and nutrients. These do add a certain amount of fat, carbs and calories but they are quite filling. I use them to make a vegetarian entrée more of a meal.
As for the whole fresh versus frozen debate, I just split the difference by eating both. The debate is about which are more nutritious. Nutrients degrade over time so the question is do you eat fresh and local, or frozen, which is frozen immediately before the nutrients have time to degrade. There is also info about how some veggies retain their nutrients more in raw form, while others release the nutrients more easily when cooked. It's all so overwhelming and indecisive that I just eat a variety and assume the best.