I don't have a favorite body part nor do I have a favorite exercise. Everyone who is honest prefers machines over free-weights, because machines are more convenient and cause less muscle pain and require less concentration and are generally less dangerous. BUT, if you like to have real gains you have to train hard and heavy, and you have to chose always the LEAST favorite exercises which actually give you the best possible results. So go for the least favorite exercises, the free weights... and go for the muscle pain!