To choose the best gas mask, you need to understand what you'll need it for. You probably know what you need if you work in the military, law enforcement, first responders, or industry. You need to be clear about your goals and how you will protect yourself against the worst.
Are you primarily concerned about a pandemic that can be handled with a P100 level filter, or do you want a military grade gas mask to prepare for more serious threats such as chemical, biological, and nuclear (CBRN), threats? Preppers with an aptitude for civil defense prepare for the possibility that a dirty Bomb could be deployed.
A dirty bomb is made with explosive and radioactive materials. This will cause radioactive particles to be spread throughout the area and pollute the air. Personal protective equipment (PPE) that is CBRN-rated will be of benefit to civilians, law enforcement, and first responders as well as the military when it comes down to a dirty bomb.
Toxic chemicals, gasses, and nerve agents
All microorganisms including bacteria, viruses, and fungi
Radioactive particles dispersed in a dirty bomb
Radioactive fallout is radioactive radiation from a nuclear weapon, device, or reactor
There are gas masks that can protect you from many threats. You should search for masks that protect against these CBRN contaminants to increase your general preparedness.