Well the ideal would be to have the bow limbs stop right at 90 at the end of the archer's draw, maybe 85 to be safe.
Ah, I think I misunderstood the question. You're asking what brace height is best? Don't think there's a perfect angle, depends on the draw length of the archer and the length of the bow...and the profile of the limbs. Lower brace heights or lower angles give the arrow more energy, but may be louder and more shocky. Higher brace is more dead in the hand and quieter, but less energy is transferred to the arrow.