Why it's on the list: Done right, split squats—particularly rear-foot-elevated or "Bulgarian" split squats—are devastating to your legs. In fact, EMG evidence suggests that 4 sets using your 10-rep max with Bulgarians create similar quad activation to back squats. The same study also found similar testosterone responses between Bulgarians and back squats!

Nevertheless, split squats are deceptively difficult, partly because of balance and partly because you're training one side at a time. And when you raise your rear foot in a so-called Bulgarian split squat, everything gets tougher. If balance is holding you back, you can just drop your back foot, hold onto a stable object, or even perform them in the Smith machine.

Split Squat Variations for Leg Development:

• Barbell Bulgarian split squat
• Dumbbell Bulgarian split squat
• Smith machine Bulgarian split squat 
• Dumbbell split squat (rear foot down)

In your workout: These work best toward the middle of a leg session. Do 3-4 sets of 8-12 reps per leg, and go for close to muscular failure on each set. If you really want to bring up the intensity, try bodyweight Bulgarians with blood flow restriction (BFR).