The reasons I believe Canada would be the easiest is because it for one is a neighboring, bordering country. Moreover, just because it is a neighboring country does not necessarily make it an easy transaction. However, historically the United States has had little to no large issues arise between the two countries. I also feel that Canada is a non-violent country and generally level headed in their social and economic beliefs. Not to mention that the US and Canada have had successful negotiations in the past. Thus supporting my opinion of why I believe Canada would be the easiest country to get a US Citizen released from prison.
References
Canada - Countries - Office of the Historian. (n.d.). Retrieved May 28, 2016, from https://history.state.gov/countries/canada
North Korea - Suspicious Minds. (2003, January). Retrieved May 28, 2016, from