According to WebProWorld the USA is responsible for the most spamming.
(Here's a list of the worst culprits:-
Here's a list of the countries who were responsible for the most spam between January-March 2010.
1. USA 13.1%
2. India 7.3%
3. Brazil 6.8%
4. S Korea 4.8%
5. Vietnam 3.4%
6. Germany 3.2%
7. United Kingdom 3.1%
8. Russia 3.1%
9. Italy 3.1%
10. France 3.0%
11. Romania 2.5%
12. Poland 2.4%
Others 47.3%
Well to me it looks like The Others are the worst offenders.)