I think you look good in your turtleneck. Most necklines seem good on you, actually. I think it's because you've got a regular size bust and torso, and your neck is not too short or long.
Here's my understanding (borne out by observations of different necklines on my own body):
- Turtlenecks visually shorten the neck and increase the apparent size of the bust. Since I don't want my regular size bust to look bigger, and I have a shortish neck, I avoid them.
- V-necks elongate the neck and "open up" space on your chest, narrowing the waist and making the torso appear less busty. They look good on most people, except maybe those with very long necks. Something about exposing a triangle of skin tends to be flattering. A button-front shirt with a few buttons open a the top also exposes a "v" of skin, which has the same effect. These -- vee necks and partly unbuttoned, collared shirts -- are my most flattering choices.
- crew necks make the neck appear shorter and the torso appear blockier and bustier. They work best on slim people with smallish boobs. I wear them sometimes, when I want full coverage, even though they don't look quite as flattering as vee necks.
- boat necks make the shoulders appear wider and the neck appear shorter.
- cowl necks are a good alternative to turtlenecks for busty women with short necks