I consider four criteria when evaluating conformation: balance, structural correctness, quality/breed characteristics and muscling. I look for a horse that's the best combination of all four.
To assess balance, I first look to see if a horse's body ties together smoothly and proportionately. Then I mentally divide him into three sections: 1) from point of shoulder to heartgirth (behind the horse's shoulder); 2) from the heartgirth to the point of hip; and 3) from the point of hip to the tailhead. Ideally, these lengths will be equal.
I want a horse's neck to be long and lean, and I want him to have an equally sloped shoulder and hip. I also want him to have a level back, and for his withers and croup to be the same height. Structurally, he should exhibit upright, correctly aligned leg bones, and hocks with neither too little, nor too much angle.
For quality/breed characteristics, a horse's head should be short (from poll to muzzle) and proportionate, tapering at the muzzle for a chiseled appearance. His eyes should be large with a kind expression. He should also show his breed's ideal characteristics.
Then I evaluate muscling on volume (muscle amount), length (how far it extends to its connection point) and definition (tone and conditioning).
