There is no such thing as "the best AP-Carry". Every champion that scales with ability power and excels in Mid-Lane is potentially op (as every other champion in this game). However it comes down to your runes/masteries, your build-order, your skilling-order and most importantly your actual skill and ability to make the most out of the skill-set given to you by the champion.
There is a thing called counter-picks, but this only indicates which one of the champions if they meet up mid will have an easier time during laning-phase, but doesn't mean that if you pick up LeBlanc vs. Annie for example you will be able to shut her down completely. You will be able to make her hug her tower and therefore not roam so effectively, but she still can get on you with the help of a good jungler and some patience.
There is a thing called counter-picks, but this only indicates which one of the champions if they meet up mid will have an easier time during laning-phase, but doesn't mean that if you pick up LeBlanc vs. Annie for example you will be able to shut her down completely. You will be able to make her hug her tower and therefore not roam so effectively, but she still can get on you with the help of a good jungler and some patience.