What would be the best vitamins for teenagers to take?

Daily adult vitamins. An over-the-counter adult multivitamin (+/- minerals) is fine for a teenager to take once a day. If a teen doesn't eat or drink much in the way of dairy products, then an adult vitamin d and calcium tablet is also needed. Some people just take a Tums smoothies once a day for their calcium supplement. Vitamin d and multivitamins are even available in "gummy" chewables for adults now.
Teen Vitamins. Make sure it is appropriate for teens, and that it has vitamin d and iron on it. Most importantly, it needs to be taken daily! put it next to something you see everyday.