Geldings. Some people will downplay their usefullness as show horses, but the truth of the matter is that they're the best thing a new user can get. Why?

They can't breed.

Having horses that can breed means there's that itch to breed them, which costs money in breeding fees. Geldings allow a user to build up a solid base of money without worrying about having to match or going broke because their one big match ended up flopping.