For the most part, I agree with that. Despite the fact that governments try to justify themselves by citing high ideals, they cannot be trusted to be in charge of this type of project for more than a short period, at best. In democracies (chosen by the people! - my favorite form of government, actually), the people in power are those who are skilled at getting themselves elected. Some have a great deal of integrity, but many of them are social climbers who are skilled at posturing and being in the right place at the right time. As for other forms of government, monarchs are usually self-serving, and wield a great deal of power. Even if one of them is a good leader, the next one in line may not be. There's not much accountability there. Theocracies expect entire populations of nations to devote their lives to serving strictly defined ideals, often ignoring the complexity of actual human needs. Certainly, there are other forms of government not mentioned here. Perhaps someone can think one that could accomplish the task at hand on a sustained basis, but I was unable to do that. Although governments should not be in change of this project, they would need to be involved in some manner, perhaps as part of a partnership, wherein they contribute some resources and help ascend some logistical hurdles.

Corporations?

What is more effectively guided by the will of the people, who vote with their hard-earned money, than the invisible hand of the marketplace? ... or so the ideal goes. Corporations are structured to make money, generally for a group of shareholders. They cannot take this role lightly, since many people's finances depend upon their shares, and this includes those who have retired and are on pensions. There is usually a small group among the shareholders who tries to steer the corporation toward socially responsible practices, but they nearly always represent a minority of the stock holdings. On the marketplace side of the system, companies make money by providing people with what they think they want and need. But people can only make informed decisions to the extent that they are provided with all of the pertinent information. Does the product or service do what is claimed? What are the unintended consequences of using this product? Is it produced or delivered using sound social practices? These are questions people might ask, but the answers are at least partially obscured, deliberately or unintentionally.

Non-governmental Organizations (NGOs)?

These are controlled by a select group of people, perhaps a board, employees, and volunteers, and may receive funds of varying amounts from changing sets of donors. Therefore, the accountability mechanism is not as balanced and dynamic as with democracies or corporations that involve large portions of the general population in their activities. But the right group of people can accomplish a great deal during the time that they remain in change. Due to the restricted accountability mechanism, however, the focus of an NGO may drift in a counterproductive direction over time, unless the people in charge are savvy about what is needed to keep it on course.

$50b to help save the world:how will you spend it?

I believe the best strategy would be to choose, very carefully, an NGO that is managed by trustworthy people, or to establish a new one. There can be no guarantee that it can be kept aligned in a productive direction indefinitely, but the right group can accomplish a great deal over a time scale of decades, through carefully-planned partnerships with governments, corporations and other NGOs.

So, what might serve as a concrete example of a possible partnership that the NGO would manage in addition to many others with varied goals that it would conduct simultaneously? Perhaps a project designed to supply drinking water to an impoverished village in an arid part of the world would be appropriate. The NGO might work with geologists and the people in the village to study the local geology. A byproduct of this would be education programs in the village school that would teach math, science and economics related to the project. Some corporation may make financial or equipment donations to help support it. Another NGO might institute an agricultural project to run in parallel with the water project. Colleges in other parts of the world could have their students become involved as part of internships. Governments would work with the NGO to arrange for visas, and might also contribute some funds. But the NGO would remain in control of the project. The goal would be for the village to wind up with a water supply infrastructure that they would have the skills to sustain, and for the remainder of the world to become educated by the experience as a case study.