The healthiest sleeping position is to sleep on your back.
When you sleep on your back your bodies head, neck, and spine will be able to rest in a neutral position.
When you sleep on your back and your body is in that neutral position there's no extra pressure on those areas, so you're less likely to experience pain.
If you sleep on your side it's best to sleep on your left side for digestive health and because sleeping on your left side helps you to poop.
Myself I start out sleeping on my back and then during the night I toss and turn and move from my back to my left side and then to my right side again and then in the morning I wake up sleeping on my left or right side.
I prefer to sleep on my back but my body moves around at night.