Western Pleasure horses are supposed to be easy to ride, and extremely laid back. Arabian showhorses are supposed to be firey. A barrel horse would make a pretty crummy WP horse.
A well trained dressage horse is a well-trained dressage horse. A well trained roper is a well trained roper.
No one discipline produces "better" horses. Look at the top horses in each individual sport, and you've got a good idea of what the 'best' horses in said discipline is. In all honesty, I don't think there is a 'correct' answer for this question, it's completely subjective.
Roro summed it up well:
dressage horses are the most well-trained for dressage, |
Eventing horses are the most well-trained for eventing,
Reining horses are the most well-trained for reining,
Trail horses are the most well-trained for trails,
You cannot accurately state that one discipline has more obedient or more well-trained horses than the other.