"Best" is a relative term. There isn't a best location, only individual preferences. Buildings 6 and 9 are closest to the A/N beach, with Building 9 being closer. Buildings 1 and 5 are the closest to water sports, with Building 5 being closer.
Buildings 1 and 6 flank the center of the resort, while Buildings 5 and 9 are furthest from the resort's center. Some people like the convenience of being so close to the restaurants, pool, etc., while other guests prefer to be at a distance from the center for a quieter location.
Our favorite is Building 6; our least favorite is Building 5. Why? Because we spend most of our day, every day, on the beach, and in 13 trips to CN, we have a particular spot that we like. It's closest to Building 6 and least convenient from Building 5.
I know everything, and I'm always right (just ask my husband).
I think all the building are fine. I think it is personal preferance. The resort is not that large. It may take you two minutes to get from end to other. We had building 5 and I liked that it was away from everything and less traffic from people walking by. If I was an AN Beach person I think that building 6 or 9 would be fine. I can't remember what building was right beside it.
And you will not be able to request a building any longer. I think when you get there you may be able to ask if there is something open if you want something different once you are there.
Our beachfront suite was in building 5 on the second floor. We loved it because it was quiet and the view of the ocean was great. It's a short walk to the pool and restaurant areas because CN is fairly small.