The best breed for you is OLD! Every breed has individuals that are calm, hot, spooky, bold etc... Honestly, the best thing you can do for yourself and the horse you buy is to look at horses that are in their teens and up. I don't care how much the owner of a 6 year old horse tells you that the horse is bombproof, it is a 6 year old horse and there will be plenty that the horse has not experienced. You want a horse that has been there and done that. A horse who has been exposed to the things you need: if you show then things like crowds of horses, trailers, crowds of people and the noise it brings etc.. and if you trail ride then atvs, traffic, dogs, water, hills, downed trees.
Take your time, don't limit yourself by breed, gender, height, or color. If the ad states "prospect" don't waste your time. Have fun and good luck!