The United States...but it depends how far back you actually go. If you go back to the Industrial Revolution times, the United States and United Kingdom certainly are among the top two. They're now being followed by some developing countries—for example, China and the like.
I'm not sure one should focus on the totals. I think more interesting is the per capita contributions to emissions. If you look at it that way, then the average North American is responsible for twice as much emissions as the typical European and about ten times as much as the typical Indian.
If you try to split out algebraically the various components, it is more important to focus on the per capita emissions rather than the total.