IMO The peak and shape of the arm is all about genetics and there is not exersize that can change that. Site injections have improved my arms a great deal from what they were but they are still nothing special.
of the bicep then concentration curls work best. there are many ways to do concentration curls but the most important thing is to make the mind/muscle connection. if you're speaking of making the arm bigger where the arm comes together then you'll need to focus on exercises that hit the full belly of the muscle pretty hard. it all boils down to concentration. genetics does play a role but it's not the only factor. i never thought i would have a peak and now 10 years later i do. it wasn't genetics either. it was years of trial and error...