An older horse is probably best for beginners, but temperament, not breed is what you want to look at. A high strung or spooky horse would be the worst choice. I would generally keep away from any horse that has ever been raced and choose a nice, uncomplicated horse to learn on. My mother just started riding last year and we got her a really lazy 11 year old tb mare. She has worked out great.
But in my experience, try to avoid a mare. Some are extremely calm, but a few will have to be medicated during a few months each year to simply keep them sane. Geldings tend to be more calm, but there are always exceptions.
I happen to love mares so don't jump on my for my last comment.