A been there done that horse that allows you to make mistakes and doesn't care. Usually an older horse is a good bet. They are the perfect teachers!
A horse with a sound mind. My very first horse was a saddlebred. Calm as he was sweet. That horse was a saint and taught me all the basics. A bit lazy though. I have seen spunky quarter horses and lazy ottbs. Breed has very little to do with anything. :)
Any horse as long as they are sound of mind, take care of their rider, are good at putting up anything, and won't do anything dirty.
I think an older animal. Mid -twenties often are the best teachers. I've a twenty-one year old half Arabian paint who love kids. He is patient and forgiving. That type of horse is worth his wieght in gold!
I would say whatever the kind of horse, but he has to be calm to have the knowledge enough to teach to the rider :)
I think Quarter Horses are great for a beginner rider!