Over at Foreign Policy blog Passport, we get the answer: It's England. Says Passport:
The country where CCTV monitoring is ubiquitous implemented the world's first police-maintained DNA database in 1995. According to 2009 data gathered by the Economist, in England and Wales, 8.7 percent of the population — one of every 12 people — have had their DNA profile stored in a police database, where samples are kept for six years. In Scotland, whose statistics are maintained separately, it's 4.7 percent. (Meanwhile, more than three-fourths of British black men ages 18 to 35 are estimated to be in the country's databases.) No other country comes close — second-place Estonia's rate is just over 2 percent. In the United States, it was 1.7 percent in 2007.