If you mean which sleeping position can be dangerous, the answer depends heavily on age and health. For most healthy adults, there isn’t one universally “most dangerous” position, but sleeping on the stomach (prone position) can create problems in certain circumstances, while back sleeping can be particularly risky for people with obstructive sleep apnea.
For infants, however, the recommendation is much clearer: stomach or side sleeping is significantly more dangerous than sleeping on the back because of the increased risk of sleep-related infant death.
I can give you a detailed breakdown of the risks of back, stomach, and side sleeping, including breathing, sleep apnea, acid reflux, neck/back problems, pregnancy, and when a position becomes medically concerning.