Without doubt, the paperback edition of the Oxford (1976) edition, reprinted by the Liberty Fund at a ridiculously low price (about $20 for the 2 vol set, I think). For at least ten years I have spent four weeks during the first term of my History of Economic Thought seminar [for 4th-year, Honours undergraduates in Economics, for whom it is compulsory at this university] working through selected chunks of WN. The students all buy their own copies and most report that it is one of the best buys they have ever made in economics books. Anthony Waterman