Which country is least likely? I'd say Costa Rica, because according to Wikipedia:
"Costa Rica was the first country to formally abolish military forces. The constitution has forbidden a standing military since 1949. It does have a public security force, whose role includes law enforcement and internal security. For this reason Costa Rica is the headquarters for the Inter-American Court of Human Rights and also the United Nations' University for Peace."
http://en.wikipedia.org/wiki/List_of_countries_without_armed_forces
Sounds like the least likely aggressors ever. Next would be Liechtenstein, IMO.