[latestrevision]

The classic, easy, example is China.

EDIT: Apparently that's a little contentious -- only 47% self-declared atheist (http://redcresearch.ie/wp-content/uploads/2012/08/RED-C-pres...), but given that 9% of those surveyed did not respond to the question, the majority of respondents to this specific survey question did self-proclaim themselves as atheists.

[CamperBob2]

Aren't most religions prohibited in China, though? Those people are basically answering the survey under duress.

[latestrevision]

Naw -- not really illegal; although it's probably politically complicated to be religious: "No state organ, public organization or individual may compel citizens to believe in, or not to believe in, any religion; nor may they discriminate against citizens because they do, or do not believe in religion." (Article 45 of their 1978 Constitution).

Even today -- decades after the state/social pressures against religious affiliation disappeared -- most former soviet states still have very large atheist populations.