I think you can say "He is the best, most intelligent person." You could also use and instead of the comma but the comma would be more standard in my opinion.
For your noun usage, you can include the second instance of the as you have it, or you could probably omit it as the meaning would be clear. If someone has a hard rule on that please jump in, as I'm not basing this in a hard rule but just feel.
So I think you can say "The best and the worst of the times" or "The best and worst of the times."
Student or Learner