Choose by character, not breed. Try out as many different horses as possible. Also, make sure you know what type of ride you prefer. I prefer a more spirited animal, while others find a spirited horse alarming. My first horse is an Arabian, BUT I have LOT of experience, especially with this horse. He is pushy, dominate, hot as hell and can be a living nightmare for 90% of people (hence why he has had so any owners). I however click with him and he respects me. While he is in no way a begginers horse, my friends Arab mare taught her how to ride hunter/jumper when my friend was six. It really depends on the horse, your level of experience, your personality and preference. And the most important rule of all is DON'T RUSH when buying.
"two things are infinite: the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein