The shade of your tint will depend on the room and the reason you want to tint your windows. To block UV rays but still allow a moderate amount of light inside, a 35% to 50% tint is best.
A 10% to 30% tint is best for privacy and security because it will only allow a small fraction of light inside. Be aware, however, that tinted windows are easier to see through at night when lights are shining from inside the house.