I don't necessarily think that there is one posture that is the absolute best, however, certain styles work better during different conditions and circumstances. I would say that keeping your posture stable and consistent is key. You don't want to look like a boblehead, and whatever you do with your lean, you want to make sure your head stays properly fixed, just like all the photos posted of the Harveys, Danny, etc.
In terms of leaning I think it comes from conditions. I grew up in the NW on mojo D1/2 tires on low grip, often damp tracks. It seems that a lot of the drivers from my area, (example: Joey Wimsett) adopt an outside lean to help account for the hard tires and slick track, as do many Brazilians who also drive on low grip conditions and chassis. Another prime example is that a few years ago, when KZ went to harder Bridgestone tires, Andrea Dale, a tall, lanky Italian, instantly started winning.
Over time the chassis adapted and others improved and Dale stopped finding as much success as he had been. In KF, Nyck De Vries had a mega inside lean and low center of gravity and it worked really well with high grip and sticky tires. I'll also attach photos of Verstappen in KZ (outside lean) and KF (inside lean). Being in the states, I rarely have dealt with any higher grip conditions, however, at times I've had to really work on keeping my body normal or even try an inside lean to account for the chassis being over stuck.
I think that naturally people have a style, and it is really difficult to change, and people find lots of success with a variety of different postures.