The answer is, in bodybuilding, you have to train everything together *(not focusing for just one muscle to be formed).
You can’t get a big chest without equally strong arms. You can’t get strong arms without strong shoulders. Everything is related.
And also, you can’t train one body part every day anyway, and there’s no point skipping the gym 4 days a week because you’re “waiting to build up” one particular part first.
But if you really want to seriously just focusing on formed one part of body, here is the recomendation :
ShouldersDelts change your overall upper body “shape” the most, and they are also the most prone to injury during all the other movements for your body. Shoulders are also a body part with three heads that all require their own movement, so training them well requires learning how subtle hand and form changes can affect which part of the delt you use.
it’s also very easy to allow other muscles to overcompensate for shoulders, so learning how to work them teaches you how to emphasize or de-emphasize other body parts during a movement. All of these mind-muscle connection strategies will then help you when building other body parts.
LegsMost people under-train legs. They are a pain to work out, the movements are exhausting, and the DOMS is rough. Working on this body part first would teach you about all the hard work muscle-building requires.
Big movements, like squat and deadlift, end up using the whole body. So training legs frequently gives a lot of work to the rest of your body too.
Additionally, Legs form a good base of stability and balance for training other body parts. Many trainer are forever having bodybuilder engage their quads and glutes during bicep curls, just to keep them stable.
And, after all explanation, this is a reminder.your core and stabilizer muscles need to be primed before anything else.
Have a fitness professional examine your posture (preferably a trainer with a corrective exercise background) before hitting any major muscle groups.
Once you've determined your muscular and postural imbalances and worked on those, should you consider starting the "building regiment".