Currently farming Sharpshooter perk (Easily the hardest out) and I thought I'd elucidate the location of the head hitbox on the different zombies-but-not-zombies:
Stalker: While invisible aim above the shimmer when she's facing you. This is the hardest hitbox to get used to because it's invisible.
Crawler: Aim where the head connects to the body. Crawlers also tend to hide behind other zombies-but-not-zombies so make sure you have a clear shot.
Siren: While the head is visually unambiguous, it's also the smallest of the zombies-but-not-zombies. Take her out at range with Xbow or just blast her down before she screams.
Clot: Compensate for their waddling walk.
Gorefast: Snipe before it charges or fire at will after charge, probably the easiest to get a headshot on.
Stalker: While invisible aim above the shimmer when she's facing you. This is the hardest hitbox to get used to because it's invisible.
Crawler: Aim where the head connects to the body. Crawlers also tend to hide behind other zombies-but-not-zombies so make sure you have a clear shot.
Siren: While the head is visually unambiguous, it's also the smallest of the zombies-but-not-zombies. Take her out at range with Xbow or just blast her down before she screams.
Clot: Compensate for their waddling walk.
Gorefast: Snipe before it charges or fire at will after charge, probably the easiest to get a headshot on.