When evaluating halter horses, I look for overall balance first, and my priorities are to find the strongest topline that carries down through a powerful hip and correct hind leg.
That said, I also look for the "pretty" in the horse. If I'm looking at mares, then I want them to be very feminine, but with stallions and geldings as well, I look for a large eye and well-proportioned, attractive head, following into a long, lean neck that ties in high and at a good angle to their shoulder and withers, as opposed to low into their chest floor.
I like a strong topline, with a short, tight back, that carries through to a powerful croup and hip. The horse must display structural correctness through its legs when viewed from all angles.
As far as muscling, I look for horses that are very expressive, as evidenced by descriptive forearms, gaskins and stifles.
When viewing the animal from behind, the center of the stifle should be the widest part of the horse, not the top of the hip above the tail. From the side, the top- and bottomlines should look like they go together, and the hip should not be substantially higher than the withers.