Even if you're a hyper-nationalist like Annoying Orange, it makes absolutely zero sense for the United States to withdraw from the UN. We are a permanent member of the Security Council, which means we are literally immune from UN sanctions and can veto whatever resolutions we want. Our presence in the UN only gives us more international power than we already have.
Edit: This is my 20,000th post. Accidentally passed it without knowing. I'm happy it was this post. Go UN!