Every plant (and every part of the plant) is going to have a different micro- and macronutrient profile. "Healthy" food has a lot of micronutrients (ie vitamins, minerals) and a generally desirable blend of macronutrients (protein, carbs [simple/complex/fiber], fats [saturated, unsaturated, polyunsaturated {omega-6, omega-9, omega 3 (ALA, DHA, EPA)}] jesus sorry for the nesting). "Healthy" isn't a goal but rather a descriptor of things that tend to promote certain goals, generally weight loss or reducing symptoms of cardiovascular disease.

Broccoli has a lot of fiber, micronutrients, and a pretty respectable net carb vs protein ratio. It's pretty much always a good idea to eat broccoli regardless of your health goals. You could say that broccoli has a very favorable calories:nutrients ratio, which you might refer to as nutrient density. An apple has more calories and less protein, micronutrients, and fiber, so you could say that an apple has a worse calorie:nutrients ratio, and is less nutrient dense.

I'd imagine that most of the benefit of vegetables comes from the high nutrient density as compared to fruits, grains, legumes, etc. A potato has less micronutrients than broccoli and over twice as many calories.