From Wikipedia (All of them)
The most effective is the side-lying external rotation, which activates the supraspinatus, subscapularis, infraspinatus and teres minor.
Lie on a bench sideways, with the arm next to the side and flexed about 90 degrees at the elbow. Rotate the upper arm, raising the dumbbell towards the ceiling to a 45 degree angle. Keep the elbow flexed, and the upper arm close to the body. Pace at two seconds up and four seconds down.
This is an excellent all-around shoulder exercise.
(from http://en.wikipedia.org/wiki/Rotator_cuff#Muscles_composing_rotator_cuff, Read its discussion also.)