The squat is one of the most functional of exercises such that it is hard to imagine a great workout without them. There are endless ways to add variety to a basic squat. One simple way to intensify the exercise is to add additional resistance such as weights held at shoulder height or even beside the body. The deeper the squat, the more the hamstring and quadriceps are involved. A squat of about 30 degrees of hip flexion turns the emphasis to the glutues mamimus. Holding a weighted bar in front of the body (close to the body) places more emphasis on the quads. I even do squats with my older older adults either with support of a fitness ball or a bar for support always watching to make sure the knee alignment is good.
The safe and effective range is 0-90% of knee flexion. At 90 degrees, the thighs are parallel to the floor. Remember, upon descending the compression forces on the knee increases. Watch out. Work within a range that is healthy for you. While the foot position of the squat has been up for debate, for some time Exercise Etc. recommends that participants do squats with a foot position that is comfortable and allows for good technique.
Resource: Angles, Positions, & Variations Exercise Etc.