From my point of view (being a Yankee fan and all), the best deal in history was purchasing Babe Ruth from the Boston Red Sox s for $100,000. Ruth went on to become the best ballplayer in history, the Yanks went on to win 26 World Series', and the Curse of the Bambino still haunts the Low Sox to this day --- ZERO World Series since that deal.
Granted, this was not a classic player for player(s) trade, but it was definitely the worst deal in sports history.