There is a YouTube original documentary (see below) that analyzed what is the "scariest thing" --- and it's conclusion was that exposure to too much CO2 (carbon dioxide) --- as one would experience suffocating with e.g., a plastic bag --- was the "scariest thing" --- in that it uniformly caused the most physiological destress/panic.
Not sure I completely agree with the methodology in coming to that conclusion... but still likely a very bad way to go.
Inert gas method as @Thanatonaut
says is suffocation, but it continually purges all the CO2 away so it doesn't build-up. The body can't detect lack of oxygen in the air, but it definitely has an adverse reaction to too much CO2.