I believe that the "passage of SALT II" is Carter's greatest achievement, for it helped continue the cooperation between the USSR and the US, as well as limiting nuclear weapons, making the world a lot more safer.
The next one would be "the signing of the camp david accords", because it allowed a mutual peace relationship between the state of Israel and Egypt. However, this in terms is not the greatest foreign policy achievement, because Egypt and Israel today are still on wary terms.
hope this helps
