I wouldn't take my word 100% on this, but I have read that that "accessory strength" excercise where you lay on the bench sideways with a dumbbell and rotate your arm like a keylock, is actually worse for your shoulder.
Pec flyes also put pressure on your RC. Don't use those.
If I were to guess, I'd say light dumbbell chest presses, beginning in the 20-30 rep range. I choose dumbbells because you can get greater range of motion (ROM) from them than a barbell. So I'd bring them down pretty low to really get the range of motion into your cuffs.
A great ROM exercise for the rotator cuffs is to take 2 swiss balls of equal size, and place them next to each other at around the width of your hand placement on a wide-grip barbell bench press. Stick your arms out to your sides, and then make a 90 degree angle with your arms with your hands pointing in the same direction as your head, and rest a forearm on each swiss ball. If you've ever done the core exercise, "the plank," your arm placement is exactly like that, only instead of the outside blade of your forearm resting on the swiss ball, you rest the palm-side of your forearm. Stay up on your toes, so it's like doing a plank, only your upper body is higher up than your lower body. Slowly roll the swiss balls outward so that your chest lowers in between them. Take this slow and controlled. Now, roll the swiss balls closer together, so you lift your chest above them. This will build range of motion; functional flexibility for activities involving your RC.
To work your shoulders from the other angle, hold a light medicine ball in your hands, and keep your arms straight. Do a lunge, working on making the step as wide as you can (also works the ROM in your hips and legs). As you lunge forward, swing the medicine ball from in front of your hips to over and slightly behind your head, keeping your arms straight throughout. Reverse the action to return. They call it a cheerleader lunge, because the way you swing your arms, it looks like that stupid thing that cheerleaders do with their arms to stick their chest out.