There are 5 basic forms of powers that a Project Manager can possess:
Legitimate (or Formal): The formal authority or power derived from the position. It is also known as Formal power.
Reward: The power of directly or indirectly rewarding the team member. Rewards may be in the form of salary, promotion, bonus or better work assignments.
Penalty (or Coercive / Punishment): The power of directly or indirectly penalizing the team members. Penalty may be in the form of suspension, reprimand, unpleasant assignments etc. It is also known as Coercive power and Punishment power.
Expert: The power derived from special knowledge or expertise.
Referent (or Charisma): The power derived from personality traits or charisma. It is also known as Charisma power.
Positional Powers vs Personal Powers
Positional powers are derived from the position of the project manager. Legitimate, Reward and Penalty powers are positional powers.
Personal powers are derived from the knowledge, expertise or personality traits of the project manager. Expert and Referent powers are personal powers.
Managers rely on positional powers whereas leaders rely on personal powers. Refer to Leadership vs Management for more details.
Best form of Power
The best form of power depends upon the situation and the maturity level of the project manager. Generally speaking, personal powers (Expert and Referent) are better than positional powers (Legitimate, Reward and Penalty).
Reference: The Powers of the Project Manager