If you can take risks, then Bitcoin is the most profitable for You. Many of us know about Bitcoin, but we cannot take risks. Without risks, no one can succeed by investing here.
I am afraid there is really no investment that comes without risk. All investments have risks, it just depends by how much. So extremely poor people really can't afford to invest in something because the money they have is only enough for their daily survival. Even if they manage their money well, it will be almost impossible.