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Scott Niedermayer, a winner wherever he goes.
A good leader makes his teammates better.
You can have a team absolutely stacked with talent but if the team's top players aren't producing, their leadership would have to be under question.
The best leaders aren't necessarily the best players. Take a look at the Washington Capitals: Alexander Ovechkin is one of the players of the decade, but since being named captain, the Capitals haven't been as great a team as they have been since he arrived. In fact, his play has declined. However, that's not to say you have to be a leader to be a good player, it's something you either are or aren't.
But there are positions that call for a degree of leadership. These are your playmakers. In football, that would be the quarterback on offense and the middle linebacker on defense. In hockey, it would be your centre or the defenseman who leads the powerplay. In basketball, it's your point guard. When these players don't play well, they can affect the entire team, which makes being a leader that much harder, and means having a good leader is important.
Now in football, measuring quarterbacks by wins has been considered an overrated statistic by many, because it is a team game and the quarterbacks don't play defense. But quarterback is a leadership position and the defense doesn't score points (usually).
If your leaders are fired up, it creates a wave effect that affects the rest of the team. That is why championship rings are always brought up when discussing quarterback greatness, because it is a leadership position. Just look at the Broncos when Tim Tebow replaced Kyle Orton. Orton is a better quarterback throwing-wise, but Tebow is a better leader, and the whole team got fired up once he started playing.
There are times when the leadership came from somewhere else, like Ray Lewis with the 2000 Ravens for example. It doesn't matter where it comes from, but it has to come from somewhere.
One recently retired leader who springs to mind is Scott Niedermayer. Niedermayer was a top defenseman for the New Jersey Devils, winning three Stanley Cups and serving as assistant captain. Then he moved to the Ducks and captained them from non-playoff team to Stanley Cup winners. Then he captained Team Canada to an Olympic gold medal.
Of course he was surrounded by talent, but he was still the man wearing the letter on his chest.