Milton's great plea for a free press in England. I found his argument quite persuasive, but I started his work already persuaded of the importance and value of a free press. Milton argues that governmental licensing of the press seldom achieves its goal, and in fact often incites revolt; that licensing of the press would result in the loss of valuable books and hence of learning, which could only lead to the diminution of the influence of England; that the men best suited to determining what should be printed would be wasting their efforts in that position and could serve the country better elsewhere; that licensing the press is tyrannous; that the truth is strong enough not to require the support of suppressing falsehood.
This is my favorite of the works I've yet read in the Harvard Classics. I'd say everyone should read Areopagitica.