Depends on what you mean by “"greatest.” If it's the most successful in terms of championships, then the NY Yankees, Boston Celtics, New England Patriots, etc. But there can be other measures of greatness, like the Dodgers breaking the color barrier by signing Jackie Robinson. Or the classy way in which a small market team like the San Antonio Spurs not only wins championships, but from the coach on down comports itself with humility and professionalism. And of course if your question transcends US sports, then Real Madrid, Barcelona, Man Utd., Bayern, would come into the mix. But if pressed, I would begrudgingly have to acknowledge the Yankees as the GOAT due to their legendary players, number of championships, market value, and fairly steady level of success for the past approximate hundred years.