Unless you reload, your best bet is the Lake City 55gr or 62gr as loaded by Federal if you can find it. Good brass to load if you ever get started or to trade off. The brass used in Federal American Eagle with the same weight bullets is usually Federal brass and most reloaders find it too soft.

Other than that, the bullet weight will depend on what you are using your AR for. The heavier weights only come into their own past 200 yards.