I have been paid to work with arabians, quarter horses, clydesdales, American Show Horses (a cross between arabians and saddlebreds), standardbreds and one thoroughbred. I have also ridden morgans and saddlebreds. My top favorites are saddlebreds, standardbreds and clydesdales, in that order.
I love the saddlebred for its beauty and animated action. There is nothing like riding a powerful, big moving horse and I prefer a horse which carries its head and neck straight up from its shoulders. While not the most intelligent (the arabs I worked with get that point), the saddlebreds I rode had more heart and willingness to please than any of the breeds I've handled. I owned and showed a saddlebred gelding.
My second choice is the standardbred, or harness race horse. I spent 13 years working at two standardbred nursery farms. On the whole, they are smart and willing. Win their heart, and they will give you all they have and then some, or die trying. I must admit, some of the nastiest individuals I've known were a group of standardbred mares, all descended from a single mare. On the whole, the breed generally has a good disposition and is fairly sturdy. If they weren't sturdy, they would not be able to stand the pounding their bodies take training and racing.
My third choice is clydesdales, after spending a summer with 6 of the sweetest, gentlest horses I've ever met. Clydes were bred to haul the heaviest wagon loads in town. Because of the feathering on their legs, they were not particularly popular with farmers. The hair, if not kept clean, can hide serious sores on their legs.