Why people are following the best leaders?

Simply because they are the best as been proven by thier increasing number of followers.

When you say 'Best Leaders' then there must be something in them that makes them best.
By immitating their attitude, behaviour and style a follower can also turn great.
May be not to that extent but he certainly gains some qualities to become a good human being.
Thus a change in attitude can bring him success in his life.
That is why people would like to follow Best Leaders.