The Netherlands, according to the Center for Global Development (CGD).
The CGD produces annually the Commitment to Development Index (CDI) where 21 rich countries are rated on “how much they help poor countries build prosperity, good government, and security”.
The CDI assigns points in seven policy areas: aid (both quantity as a share of income and quality), trade, investment, migration, environment, security, and technology. Within each component, a country receives points for policies and actions that support poor nations in their efforts to build prosperity, good government, and security. The seven components are averaged for a final score. The scoring adjusts for size in order to discern how much countries are living up to their potential to help.
CGD's Commitment to Development Index 2006 was recently released and has the Netherlands on top, with Denmark, Sweden and Norway following closely. A lot of interesting information included in the site.