what i know best is software development, but it's a hard marketplace to enter. if i had the resources, i'd hire a small team of developers and some sales people. the key is relationships, so i'd look for technical sales from high up the food chain at major industries.
but failing that, i'd probably start a company to provide water and electricity to africa. if we can make it easier to live over there than i think there'd be a fantastic net effect on the rest of the world.