8. Kevin McHale
The aforementioned trade that brought Robert Parish to the Celtics also resulted in them selecting Hall of Fame big man Kevin McHale. It's rare that a trade nets a team two future Hall of Famers but that deal did and it helped to bring about another Celtics dynasty. McHale was known as an elite low-post option who was remarkably efficient during his 13-season career, even leading the NBA in field goal percentage back-to-back seasons while averaging better than 22 points per game.
Factor in that he was a seven-time NBA all-star, a two-time Sixth-Man of the Year winner, and made all-defense six times and the Warriors were probably kicking themselves for not having the foresight to take Mchale and not holding onto Parish. Instead, with McHale, along with Larry Bird and Parish, the Celtics went to five NBA Finals and won three championships, all in the 1980s.
On the other hand, the Warriors made the playoffs just twice during that decade and only four times over McHale's entire career, a result of some questionable decision-making. In fact, it would take more than 30 years since the fateful trade for Golden State to win a championship, despite having several great players come and go.