There truly is not a "best" sitting posture. What I prefer you to think about is how you are moving through the positions while you are sitting. So the best sitting posture is the next one. The goal is to find small micro-movements throughout the day, so while you are sitting, make sure you are changing your position.
If you notice that you tend to lean to one side quite often, than your goal is to start moving your body to the opposite position, which can be really tough because it isn't a comfortable position for you since you are rarely sitting like that.