Dachau, eldest of all the camps, the only one to be run continuously from its inception in 1933 to the end of the war. The first wave of camps was run by the SA, and all of these were shut down within the year, but not Dachau. It was run by the SS, at the time a very small organization. What happened there was so monstrous I shall not pollute this site by recounting it. Those curious about it - and you are warned - may be interested in Nikolaus Wachsmann's
KL - A History of the Nazi Concentration Camps.