Copyright date or publication date, which one should I cite?

I have actually asked a librarian about this issue as it came up for me in the past.

She told me to cite the exact copy that I am reading. If what you are reading is the first paperback edition (usually the inside left initial pages will have information about this), then do cite that.

The purpose of giving details in the citation (rather than just saying "according to Joe Bloggs") is to allow other people to find the cited work. Year of publication is something that could plausibly help somebody find the work; copyright date is not, so cite the year of publication.