Back mostly, but some side. Only a little stomach. Back needs a thick pillow, side needs less, and stomach needs almost none.
Stomach is odd. I can lay flat and it is comfortable, but only if the head is turned to the left. Neck must not turn as far to the right as it does to the left. Can turn slightly up if I am holding a pillow in my arms.
Its tough because I like to stick my arms out to the side, but since I started sharing a bed, my wife decided she doesn't like arms smacking her in the face in the middle of the night.
The big determiner in how much I can sleep on my stomach is how much water I drank during the day. I feel better, and my back is less stiff, in the morning if I am well-hydrated.