Foreign Policy magazine will tell you it is the Netherlands. If you click on the link, you will see their rankings. (Note that the paper edition has more detail and less confusing visuals.) Denmark and Sweden are next in line, the U.S. comes in seventh. Japan comes in last among the developed nations. The metrics are adjusted to per capita terms.
Unlike many other studies, this metric goes beyond foreign aid payments. It also considers immigration policy, trade, and investment as means of helping the world’s poor.
Looking at the index, the aid component is too heavily weighted and the immigration component is underweighted. I find the issue-by-issue scores most informative. Canada has the best immigration score, the U.S. is a close second. The U.S. has the best trade score. The Netherlands and Germany have the highest investment scores. The Scandinavian countries have the highest aid scores, by far. Norway has the lowest trade score, largely because of its high agricultural tariffs. By the way, here is an earlier MR post on the importance of remittances.
The bottom line: OK, I am an American. But it is hard for me to resist pushing the United States into the top slot. Can we look at all of history since the 1930s? If you consider military and cultural influence over the long haul, I think we have it clinched.