The dream that would have made Willy happy is to have followed his inner love for building and creating things. It would have been a gift that kept on giving, since it would have given him both the pleasure and the benefit of being productive in society, and to be there for his children.
Instead, Willy's dream was to follow the steps of another man. When he was little, a man named Dave Singleman. This man was the inspiration behind Willy's career choice. He saw how this man supposedly made lots of money out of a hotel room. He figured he could do the same, and have a good lifestyle just on the basis of "being liked" and achieve financial success.
However, when Biff talks about a "wrong dream" he is mostly referring to the fact that his father was following another man's dream. He never once stopped to figure his own destiny and, instead, went following a journey already walked by another person.