You probably want to work the whole of your legs; your weak leg will catch up. Nevertheless, to add focus to a weak leg, the best lifts are on machines that 1) work the muscles you want to strengthen, and 2) allow you to work one leg at a time. For your situation, to strengthen a quad, I would comtinue with the squats, and possibly the lunges but add seated leg extensions for the left leg to strengthen that quad. For balance I would add lying leg curls to strengthen the hamstring on the same leg. And for kicks, you might want try all of the leg specific machines at your gym and assess which might give you best response.