Any answer to that question will be correct, because any breed can produce easy or difficult animals.
The best working horse I ever had, bar none, was a QH mare. It was like she was born already knowing how to work the cattle. The smoothest and most fun riding horse I ever had was our TWH stallion. My current older Friesian/Saddlebred is certainly one of the 5 easiest horses I've ever started under the saddle, but also one of the most head strong. The Percheron that was the horse I was taught to ride on (forever ago
and we didn't own) was easy going, but also a bit of a dead head which almost certainly contributed to it being a good horse for a child to learn to ride on.
One of the most difficult horses I ever dealt with a draft mix and another was a QH, but they both turned out good horses in the end.
Horses are like people and the breeds are like ethnicity or nationalities. You'll find some of everything in any of them.