Leg development reigns king when it comes to putting on mass and getting your metabolism working hard. You won't find many big, strong people in the gym who don't regularly occupy the squat rack and leg press machine.
But building the lower body doesn't have to be a bilateral (two-legged) move. We now know that you can make massive gains with unilateral (one-legged) exercises. Thanks to coaches like Mike Boyle and Ben Bruno, single-leg work is more widely recognized as an option for not only building athletic capacity, but for maximizing size potential.
The legs are equipped with too many muscles to solely rely on bilateral work. Abductors, adductors, and gluteus medius are just a few muscles that don't maximize their potential without stabilizing the leg on its own. To tap into that potential and push your leg development, do the single-leg hex/trap bar deadlift:
This variation keeps the back leg resting on the hex bar to create what's closer to a closed-chain movement. Keeping your leg back emphasizes rigid form and the ability to sit all the way back into the stance.
While maintaining a strong posture, stand from the bottom position, ensuring that your arms are completely perpendicular to the ground. Descend with the hips moving back until the bar meets the ground.
Try this out for 3 sets of 8 reps to start. Increase reps before sets when progressing forward.