It depends on the circumstances, but I try to get a better position when I can. I probably shoot more game kneeling using a convenient tree/sapling as a rest than any other position, but the last deer I killed, just recently, was shot from prone. She was lying down with her head up looking towards me, with low cover obstructing her body from view and leaving only head and neck visible, down a clear cutline nearly 200 yards away, and I shot her under the chin into the top of the neck. Others recently included one offhand into the back of the neck as it galloped away, and another shot while I was standing braced against a tree at about 90 yards.
One position I used to use a bit, but don't much anymore, is sitting. I'd only really use it if I have plenty of time and can't go prone due to cover or terrain. Dropping to a low kneel (ie sitting on the side of my right foot) with a convenient tree to brace up against, is easier and faster to get into and usually steady enough.
