There is no such thing as the perfect horse. However, how the pieces of a horse “fit together” can significantly influence how that horse performs and help that horse stay sound throughout its career. For any discipline the overall balance of the horse is important.
However, when you start looking at different disciplines, you look for horses with conformational attributes that will make their job easier. For
example a horse that excels at Western Pleasure is likely to look different to a top level dressage horse or a jumper. The western horse will likely have his head and neck set lower onto his body than a dressage horse, who will have a more upright
conformation and will likely have a shorter neck that ties in higher. A jumper, or event horse, will likely have a longer neck (as that is useful for balance over fences).
One of the best resources that I’ve found on understanding conformation is Judy Wardrope’s, JW Equine. She has several articles that highlight the conformation attributes that most suit specific disciplines. The links to the articles below are PDF files.
I also found this video from eventer Phyllis Dawson to be helpful.