I have a 4" x 6"-16' treated wooden beam. I am trying to determine which way the beam should be placed to provide the most strength for the decking.
Always for the most strength, place the beam so the 4" faces are the top and bottom and the 6" dimension is the height.
As a mention I hope you are planning on adding posts or some means of support in the center. A 16' beam supported by the ends only will not hold up a deck.