What is the best exercise for quads? It seems that no matter what I do, I don’t feel like I worked them hard enough. I do squats, lunges, leg extensions, etc. Is there anything that would concentrate on quads only?

I am a fan of Hack Squats to really target the quads and well as the 45d Leg Press. I would be aware of your foot position and make sure that for example, on squats, you are really targeting the quads, rather than the posterior muscles that can do a lot of the loading when your feet are too far up. The same for the leg press. Make sure your legs are not too-far forward on the platform. Form is crucial. Look at your diet and make sure that on leg training days you are getting enough calories t support muscle growth. Good luck ; )