The best long-term perfect posture is subjective due to the variety of body types, incorrect posture differs from person to person and person's proper posture can be incorrect posture for someone else and vice versa.
Usually the good sitting posture can be determined by the following methods:
Support your back to avoid back pain.
Adjust your chair.
Short answer: there is no best long-term sitting posture. If there is any, take care of posture (I'd risk to say upright is best), and move every certain amount of time in the chair and/or around.
I'm for positive responses so let me elaborate a bit more. All cautious considerations, as kenorb properly states, are ok. However, even if some of those ...
There is no brace position because you will not have warning of a car crash. Think about it. If there was warning, why would there be a crash? The driver would use the warning to avoid the crash. Unlike a pilot who must land the plane eventually regardless of conditions, a car driver can simply stop the car and get out.
If, under some hard-to-imagine ...
From my experience, it is better to change postures while you sit long.
Most often, I sit in a posture similar to below yoga posture 'Padmasana' in my chair, but not for so long, just for 5 or 10 minutes. It really helps to reduce the tension on the back.
Actually, it need not be in exactly this position, just cross your legs as you can. After 5 or 10 ...
I can share my personal experience with you. I'm a 31 year old guy with a sedentary job where I need to spend hours on my computer. I developed lower back pain around 6-7 years ago and more recently upper back and neck pain. I went for Physiotherapy and felt relieved from the pain. So my answer will be based on recommendations of my physiotherapist.
Per this NPR article in a meta-analysis it seems it's not yet clear:
there isn't really any evidence that standing is better than sitting,
Carr says the finding of the Cochrane review doesn't mean that
standing desks and variations are useless. It just means there hasn't
been enough study of the desks to say either way. "The ...
Aren't you in luck, I asked a very similar question over in physical fitness SE:
Here is the top answer:
A recent meta-analysis of 23 published studies is MacEwen, MacDonald,
and Burr, "A systematic review of standing and treadmill desks in the