There is really no ONE best exercise for a particular muscle group. There are some exercises that are better, because they work a particular muscle through the full range of motion and some that assure the highest stimulation of all muscle fibers. However, to truly give any muscle a nice, well defined shape, you have to work it in many different ways and especially at many different angles, choosing exercises that will use all the functions of the muscle in question.
Also, performing exercises that require a high level of stabilization can be very helpful in getting toned in a nice, even way, because you will be incorporating many different muscles around the one you are trying to work, and therefore getting a better challenge overall.
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