This is the best exercise for your deltoids. It works mostly the medial head. I usually prescribe routines based around the medial and rear deltoids. The anterior head is involved when we work our chest so, I prefer to let it rest. Obviously, every client is different.
Stand holding your dumbbells at your sides, not in front of your body; elbows relaxed, palms facing your body.
Exhale and slowly raise your elbows up and out to your sides. Your elbows and upper arms should rise together and lead the movement ahead of the forearms and dumbbells. Pause at the top and descend slowly.
Inhale, back to the starting position and repeat.