In cites just published an overview of the top ranking countries for science over all fields. The report is based on the Essential Science Indicators it is therefore based on the past 10 years of publication and citation data collected from the Thomson ISI covered journals.
The first two tables rank the countries by accumulated citations and published papers. Most interesting however is the third table in which the countries are ranked on citations per paper. After omitting the smallest countries from that list we see that Switzerland (14.32 cpp) is leading the list followed by USA, Denmark and Netherlands just in the 4th postion with 12.85 cpp. Scotland, Sweden England, Finland, Canada and Belgium complete the top 10.