I said "none of the above" because the question seems to imply these organizations are healthy and properly constructed as they stand.
The United States should be engaged in the world, and should seek a benevolent stance regarding the world. That much is merely what any would expect from any of his neighbors, and thus it what he should offer to any of this neighbors.
However, the United States is a sovereign nation. Within our borders, US law is the highest law there is, to be superseded by none. We owe no fealty to any higher entity, no duty of obeisance to anyone but ourselves; this is the nature of all sovereign nations. We are free to do as we will; none may command us to do otherwise.
Organizations such as the UN, and treaties such as Kyoto, seek to constrain and intrude upon that sovereignty. They seek to address not merely how nations should handle affairs beyond their borders, but within their borders--and no nation is obligated to surrender their sovereignty in that fashion.
Others, such as NATO, serve a purpose whose time may very well have passed, and it is not improper to question whether such an organization should be disbanded in favor of a more timely and relevant construct (although I stop short of claiming to know what that construct might be).
The US should be engaged in the world. It should be engaged as a sovereign nation, free and beholden to no one. It should preserve its sovereignty above all else--for any nation, defense of its sovereignty is the supreme civic virtue.