Ask HN: what's the best business you've ever seen?

A friend of mine asked me an interesting question last week: what's the best business I had ever seen? The point was to make a list of answers about the "best businesses" and then gauge whether one could discover a common theme of how to find and address market opportunities.

By "best business", I don't just mean highly profitable, but also the elegance of addressing the market need, the scalability of the solution, the ease/capital-efficiency of launching and running it, the potential positive external effects it has.

We came up with a list that was as wide as Google, Kiva and Cash4Gold. What's your preferred "best business" and why?