What's the best treatment for dark spots following acne that healed?

Nothing: Please forgive me. Trying to bleach each little dark spot will be frustrating, largely ineffective, potentially disfiguring, and cause you to spend too much time looking at your face when most people care more about how fit you are, how you move, what you have to say, what you can do well, and how you treat others. Leave them alone. They'll fade over the months.