All the components of the conducting system can generate an action potential for the contraction of heart muscle, but the sino-atrial node acts as the pacemaker because:

The sino-atrial node has a higher inherent rate of depolarisation. Autorhythmic fibers in the SA node would initiate an action potential about every 0.6 second, or 100 times per minute on their own. Thus, the SA node sets the rhythm for contraction of the heart. This rate is faster than that of any other autorhythmic fibers. Because action potentials from the SA node spread through the conduction system and stimulate other areas before the other areas are able to generate an action potential at their own, slower rate, the SA node acts as the natural pacemaker of the heart.