Many People have trouble executing barbell squats form correctly. If this is you, then you may be risking injury to your knees, hips, and back. But, when you have excellent barbell squat form you will stimulate strength gains for your entire body.
Doing single leg squats is a great way to isolate one leg at a time, but can be very difficult to execute. The side lunge provides a great base for balance while isolating the same muscle groups as a single leg squat.