The open data index ranks countries by how much data they make publically available based on 10 areas (transport, budget, spending, etc). It isn't a full global picture and it doesn't cover all datasets in every country. However, it's still useful as a comparison tool and a way of seeing what data is (and isn't available).

They have rankings for 70 countries. The top 10 (from October 2013) are

  1. UK
  2. US
  3. Denmark
  4. Norway
  5. Netherlands
  6. Finland
  7. Sweden
  8. New Zealand
  9. Australia
  10. Canada

https://index.okfn.org/country