Most bodybuilding magazines extol the virtues of the back squat to build big quads. The problem is that the back squat usually isn't an effective quadriceps builder, unless you have a specific skeletal structure (short femurs, long torso) and sufficient mobility to go along with it. Here's a simple, basic test to determine whether or not the back squat is right for you.
Stand next to a mirror. Start with your arms crossed at your chest with your feet slightly wider than shoulder width. Drop into a full squat while keeping your torso as vertical as possible. If, in the bottom position, your knees are fully flexed, your heels are on the ground, and (this is imperative) your torso is more than 75 degrees relative to the ground, you're built for the back squat. Your bottom position should look like this:
Tall people with long femurs, or those who lack mobility, end up shifting too far forward to overload the quadriceps. This doesn't mean the back squat won't be beneficial, but it must be understood that when you can't achieve the depicted position, a back squat won't build big, strong quads as quickly as other lifts.
Assuming that you're shorter than 6'2", and you don't have femurs like a pro basketball player, you might just need to work on your technique and mobility. One effective exercise is the wall squat.
- Stand facing a wall with your nose and toes against it. Your feet should be slightly wider than shoulder width with your feet angled out very slightly.
- Squat down as far as possible with your arms hanging between your legs. (I prefer this to the variation where the arms are kept high.) With each rep, you should be able to drop a little farther.
- Perform 3 sets of 15 reps every day until you can drop your hips below your knees.
For those who can't drop their hips below their knees after performing the wall squat every day for two weeks, focus on the front squat and single-leg squat for quadriceps development. The wall squat, however, is great for anyone. It'll really hone your squat and deadlift technique, regardless of your skeletal structure or stature.