IMO the best is to occupy the center of the line to improve your visibility (especially when the road bends right) and to give the cars some incentive to slow down and overtake more on the left and then move closer to the right edge when they're actually overtaking you. This breaks down if the traffic is too heavy though.

The exception is on left bends. There it's best to stay on the right, for three reasons: better visibility, you can have a straighter trajectory with a higher speed and if there is only one lane in each direction, cars going too fast in the opposite direction may cross into your lane.