These bodyparts you do not see a lot of bodybuilders working them.
The Neck: I do not know why more bodybuilders do not work their neck, there is nothing more impressive than a thick neck on a massive pair of traps.
Front of the calf: If you have shitty calves then you should be working the front for a more impressive looking calf.
The Butt: Some male bodybuilders do not like to work the butt machine because they think its so gay, but a nice ass in a pair of tight fitting jeans looks good not only to men but to women too.