which leadership style is effective today??

Leadership is all about communication, the style can be autocratic or democratic. If they are communicated wrongly in the behaviour and/or language of the leader, both would eventually lead to discomfort. I would put more emphasis on communication by the leader to his team and organisation, such that whatever the style it is not construed as wrong or ineffective. The leader has to keep moving between these styles depending the decision on hand and the complexity involved.

It all depend on the industry or the field you work in, if you work in a high-tech. technical environment were creativity is a vital issue, from my experience leadership is achieved  by means of open and honest communications (Participative).

According to the basic human psychology always the participative(Democratic) used to create the Win-Win situation which is widely accepted nowadays through out the world. And a best leader is "One who knows way, One who shows the way, and One who goes the way". So if a leader doesn't want to shows the way and to be authoritarian who will like to follow that leader. And all that efforts unrecognized wiill go waste therefore always we the human mind need recognition for anything it is doing. Authoritarian always don't recognize, therefore the style doesn't work.

This style involves the leader including one or more employees in the decision making process (determining what to do and how to do it). However, the leader maintains the final decision making authority. Using this style is not a sign of weakness, rather it is a sign of strength that your employees will respect.

This is normally used when you have part of the information, and your employees have other parts. Note that a leader is not expected to know everything -- this is why you employ knowledgeable and skillful employees. Using this style is of mutual benefit -- it allows them to become part of the team and allows you to make better decisions.