North Korea have a network of internment camps where regime opponents are imprisoned - nobody really knows what the imprisonment rate is, but estimates[0] are 250,000-300,000 people.
On a per capita rate, they would be about even with the United States (around 700 per 100k[1]) at the top end of estimates - which is worrying considering North Korea also practice a policy of "three generations of punishment" where entire families are collectively punished and you have children being born in camps.
The land of the free and the most totalitarian state in the world with a horrible history of human rights abuses - it really shouldn't even be a contest as to who is worse, but unfortunately it is.
[1] Doesn't include territories, juveniles, military prison or immigration detention. Including those on parole or probation it increases to 3.2% of the population.
Though I don't see anything definite on prison rates. Hard to get precise info out of N Korea. This absolute dismal circumstances the prisoners suffer is clear though.
They hand out 25 year sentences like candy.