(The guy who asked me this was actually asking me about chest muscles, but this answer pretty much applies to every muscle group.)
There is actually no single best exercise for any muscle group. In fact, it’s not actually very beneficial to do the same exercise/s over and over. Your muscles have memory, and if they are always being worked in the same way, your body starts to know what you are going to do before you even do it, it becomes no shock/challenge to your body, your results will begin to plateau and you are more susceptible to injury. (This is one of the reasons crossfitters often end up injured – doing the same movements and exercises over and over and over.)
Instead, using a range of different exercises to target said muscle group from all directions/angles, will give a far better result. Let’s use chest as an example. If you only ever do bench press, your muscles are only ever going to be strong when you’re trying to push something straight out in front of you. But what about if you lift something off the ground, or push something downwards, or push something from left to right, across the front of your body? All these movements would require use of the chest muscles, so wouldn’t it make more sense to be strong from every angle?
There are 3 planes of motion that you can work through, sagital (forward and backward), frontal (side to side) and rotational. Depending on what you’re actually trying to achieve from working out and how often and how much you’re targeting a muscle group, you don’t necessarily have to work through them all in the one single workout. But to gain maximum results and functionality, over a period of time you should always MIX UP your exercises to target the muscles in different planes, angles and directions.