I think these days, really, in all the popular bloodlines, you can find examples of gorgeous heads. You can also find some rather plain examples, and some ugly ones, too
What I think is pretty when it comes to miniature horse's heads is a short head, refined muzzle, and large eyes. I also like to be able to tell by the face if it's a "boy" or a "girl"
In my own herd, DunIT has put gorgeous heads on all his foals. DunIT's likes are BTU, Yellow Diamond Little Feather and EK. His foals are also very necky and upright, which I like even more than a pretty head
Some examples of what I think our our prettiest headed horses, and their line...
DunIT, mentioned above:
Some a couple of DunIT's get:
Destiny, a grandon of both Buck Echo and Double Destiny:
Hope, a daughter of Cross Country's Rowdys Reflection (Rowdy Lines):
A BTU granddaughter, Double, with a short beautiful head:
Heavily Buckeroo bred fillies with gorgeous, short heads (all out of BTU Mares, first two sired by Destiny and the third by Big City, Erica's Buckeroo son):