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