When it comes to building massive shoulders the key is strategic overload, eccentric muscle damage, and time under tension. The eccentric accentuated (which means lower slowly) push press requires each of these hypertrophy-inducing mechanisms. That makes it one of the most effective functional mass builders for the entire shoulder region including the delts, traps, and upper back.
Simply do a standard push press by using both your low body and upper body to drive the weight overhead. Pause in the top position for several seconds, then slowly lower the barbell for 4-5 seconds. Pause in the bottom position before repeating this for 3-6 reps.
Because the load should be 15-25% greater than what you could typically handle with the strict press, doing these with a controlled negative will increase the growth-inducing benefits. Think of it as heavy negatives for the shoulders but instead of having a spotter help you lift the weight up (like you would on a negative bench press set), your legs will assist instead.
Besides the hypertrophy stimulus these create, using inordinately heavy leads with strict motor control and rigid body mechanics does wonders for strength and neuromuscular efficiency. As a result, your numbers on just about every upper body exercise should dramatically improve, which will have a direct impact on building gigantic shoulders.
Due to the extended time under tension (each rep should last a minimum of 8 seconds), a full set should last about 30 seconds. Using heavy loads in an overhead position for such an extended period of time triggers massive gains.
Want to up the challenge? Do your final set then drop the weight in half and add more strict reps to failure. The discomfort will be almost unbearable but the resulting shoulder growth will be well worth the pain.