this is actually a very simple question if you do not try to over complicate as some of the book instructors seem to do.
the best lift drag ratio will always occur at the bottom of the total drag curve. beware of explanations and diagrams that just show an aerofoil. bolt on an aircraft and an engine and things change.
to fly the a/c straight and level is going to require thrust and this in itself will cause a different amount of total drag than that in the glide and will therefore change the total drag curve.
thinking about a glider always helps, the speed is the same for the glide and the cruise!!!!!!