One of the most important functional exercises to master is the squat. With squats, you can work every single muscle chain in the body, and do it in a functionally correct manner with proper posture and motion.
Moving/Exercise stimulates mitochondrial replication and "up-regulates" many genes in the body. Healthy motion is even more important than good eating habits (although you can't have excellent health without either -- not for long!).
Here is an excellent example of 20 different types of squats. My daily goal is to do 100 near-effortless squats. Currently, I do about 50 full squats each day.
The average person, once "re-awakening" and restructuring the lower back/leg muscle chains, should be able to do about 15 full squats without any "burn". This indicates good/average energy conversion and re-conversion in the muscles being used.
Wall sits are another great exercise, but I find doing them more frustrating and not very enjoyable.
For hip and leg function, I also do standing lunges... I do sets of 15 with 15 added pounds of weight. I am not into bulk, I'm more into function and form.
However, if a person has been sitting for a long time (years) and is not used to exercise, a condition known as a "sleeping iliopsoas" can cause serious injury with even one single lunge. It's better to make sure all of those muscles are working properly before doing them, even without weight!