As one would expect, US is the leader with 2539 medals total (both in Summer and Winter olympics), with Soviet Union second (1204), France third (745) and Italy fourth (643).
The summer olympics medal count ranking is the same as the "total" ranking above, with US first (2190 medals: 896 gold, 690 silver, 604 bronze). It's followed by 2) SOviet Union, 3) France, 4) Italy and 5) Great Britain.
In Winter Olympics, Norway is the best, with 280 medals (98-98-84), followed by the US, Soviet Union, Germany and Austria.
This link is really useful:
http://en.wikipedia.org/wiki/Total_Olympics_medal_count