If you do focus on the biceps and triceps, outstanding results can follow. Showing those 19" or 20" cannons will blow most people away. But if genetic limits are in place, those extra high peaks ( so high that they have snow on them most of the year) can never develop. You may develop impressive hills but not the Alps. If your bicep inserts are longer and nearer the elbow joints than the chances are a flatter and/or wider muscle is the result. Further away from the elbows bicep inserts can give a potential for a much higher peak. If a higher impressive bicep peak could be developed by hard training time (and a little chemical help) than all the Pro's would show that development. Lot of 20"/21" arms out there, full and well rounded but minus that higher peak.
Like Columbo and his low and wide lats. A genetic reward without a doubt. Scott is a perfect example of a man who had much less to start with than most other future BB'ers. Medium boned, narrow shoulder/body frame Through shear effort and planning he became one of the all time greats. That second pic of him (from the top) supplied by Pumpster, is just amazing. Must have been at the time when he did have a true 20" arm, for a short period anyway. Gironda style Preacher curls (for the most part) and making the most of hidden genetic potential that took him from a good BB'ing arm at his beginning, to the extreme development shown in latter years. If he had been born with longer bicep inserts, than perhaps that arm would not have been quite that impressive.
Guy's who trained with a variety of bicep exercises seem to have the advantage in the outstanding arms department; machines, cables, compound movements, etc.. People I've actually seen and talked to tend to favor a cheat style BB curl, incline hanging DB curls and Scott bench, one arm DB curls. But they all seem to go back to the BB curl, from time to time, as one of their favorites. That higher peak just seems to come with their training, no matter what type bicep movement they are doing. That is ,if anyone is blessed that way. Good Luck.