Personally, I think that prisons need to be both retribution and rehabilitation at the same time, and base their mode of operation on the Military, but that's my personal opinion. Based on that I'd say the Japanese probably come the closest, with the actual Military being second. I say this not because I don't care about prisoners, but because I know first hand that the way bootcamp operates is a fast efficient way to retrain people who are unwilling to be trained. It also doesn't do this with violence (anymore) so it would be humane without being "soft".

If however you think prisons should be about rehabilitation then, I believe Denmark or Sweden have the best. They also have a highly homogeneous society with only one main race to deal with, so that's why that works for them. In other words, it's easier to retrain people to fit into society if everyone is already from that same society.

Other than that, they just suck everywhere.