Which has more caffeine black coffee or Mountain Dew?

Speaking from the big picture, probably not.

It's loaded with sugar, which puts the body into overdrive followed by a crash; and it contains caffeine, which pulls fluids from the body and can actually (and ironically) lead to dehydration.

What Mountain Dew (and other similar products) are good for is providing a short burst of energy, and satisfying a craving for sweetness and/or calories.