The answer to this question goes far beyond than a simple answer sort of "Michael Jordan" or "Scottie Pippen" The Bulls create an unstopable offense when they execute their triangle offense. This offense, as the name says, consists of having three players into three specifics spots on the court. All five players keep moving continusly, but at the time of the shot the three needed players are always there.
On the defense, players like Michael Jordan, Scottie Pippen and Dennis Rodman are the main armor of their team. First, Jordan, the guy who is fast, who can block and who can steal the ball. He is everywhere in the court. That is why opponents fear him. Second, Scottie Pippen, like Jordan he can steal and block. He is the type of guy that will give his life in order to stop the opposition. Lastly, Dennis Rodman, the ultimate rebounder. He can get rebounds on both ends of the court, "because you can not win without the ball.
If the question above is still not answerable? well, that is because we have not mentioned the bench. The Bulls have awesome players as starters, but when ever they get tired, their playing rythim can continue the same. Players like Tony Kukoc and Steve Kerr have their own special tricks which help make the Bulls an extraordinary team.