So, that's a good question actually!
There is actually no such thing as a one-size-fits-all best posture :)
Your best posture is the one that fits your body best, for a specific task and requirement. Your own body generally arranges itself for the best posture for the task, arranging your limbs and positions accordingly.
However, our body is inherently designed for efficiency, it's trying to save energy, so it won't always pick the best option for us long term.
Slouching for a long amount of time, not getting up often enough, those are just some things we naturally do that may feel great short term, but can be harmful long term.
So, to sum it up, your body generally picks the best posture for you short term, but be sure to change positions and move often and well to stay healthy.