I was going to say 'Singapore' until I read people's comments about 'teachers trump curriculum'. GS9's school uses Math Investigations and my complaint is it is too slow for a gifted child. We use Singapore for afterschooling and I'll let him do a chapter test before we do the chapter. If he already knows how to do it, there is no reason to cover the material. If he knows it, and needs practice, I handle that a couple ways. I'll usually give him a couple problems to work for practice, and simultaneously push forward. That gives him practice as well as rewarding him with a new challenge.