The Best Sitting Position at a Desk Is One You Can Change
The best sitting position at a desk gives you support without trapping you. Set the chair and screen, then change position before you feel stuck.
Set your feet and hips first
Place your feet on the floor or a footrest and sit far enough back that the chair can support you. Your knees can be level with or slightly below your hips if that feels comfortable. You should still be able to shift your weight.
A chair is doing its job when you can work without constantly bracing or sliding forward. If it forces one shape, use small position changes instead of fighting it.
Bring the work to you
Keep the monitor in front of your body and arrange the keyboard and mouse so your elbows stay close. Put frequently used items within reach. Repeated reaching is often a setup problem wearing a posture disguise.
OSHA recommends neutral positions for computer work, while Mayo Clinic points out that even a good setup needs regular breaks.
Leave the position often
Stand, walk, or stretch your arms at regular points in the day. The best sitting position becomes less useful if you stay in it for hours without changing anything.