The delts are usually described as having three muscle heads – anterior, middle, and posterior – but there are really SEVEN individual segments. The delts also have a mixed fiber type, so you need to hit them with both heavy weights and low reps as well as using higher reps with lighter weights. But what's the best overall exercise? The standing dumbbell press.
For overall muscle activity in all three heads of the delts, there's one exercise that comes out on top, and it's probably not what you would have guessed.
Saeterbakken, et al. (2013) investigated four different types of shoulder presses and found that muscle activity was the highest (15% higher) in the standing dumbbell press and not the standing barbell press.
So if you only have time for one shoulder exercise, make it the standing dumbbell press.