Generally speaking, side or back sleeping is the best. I say this because true stomach sleepers must wrench their necks to either side. When you are a small child, it works because you can turn your head 90 degrees to either side. As you get older, this becomes more difficult and you can end up with a strained neck.
However, with a good neck pillow that opens up the sinuses and restores the neck curve, apnea and snoring are much improved. With a slight incline, reflux disease can be improved also.
A neck pillow also keeps your head level when youre on your side. Put a pillow between your knees and you keep your legs and pelvis level too.