What is the best fuel additive for car?

Buy them all and mix the entire lot into your fuel.  That should be the optimum right?  Bet you a $100, your car will not perform any better.  It is better than pisssing in the tank.

No such thing just buy better fuel from the pump.

NO SUCH THING. The engineers that design the engine work with the refiners that formulate fuel. The ONLY additives worth the price are ALREADY there.

best fuel additive is the fuel itself .

................More gasoline.

No fuel additive at all.

That depends on why you want to add a fuel additive in the first place. If you have water in your gasoline buy a can of HEAT. If you've been told your fuel injectors need cleaning because you let the vehicle sit for over 6 months it's better to have a mechanic run a special cleaner such as the stuff 3M makes in a pressurized can that takes special equipment to hook up to the fuel rail. If you want to pour a cleaner into the gas tank Techron makes the bet fuel additive for that. If you have a high compression motor and can't find premium high octane gasoline buy a bottle of octane booster. 

You probably don’t need any as modern fuels have detergents added which keeps your engine clean. I remember in the 60s my Dad taking his car in for a “de-coke”. Have you heard of anyone doing that for about 40 years?

The only exception is for diesel in really cold weather, -15Cor lower. You can get additives to stop it waxing ( modern diesel isn’t as quick to form wax as it used to be), which will block the fuel filter and stop the engine. I experienced this in 2010 , when we had temperatures down to -20C where I live. Alternatively , use a synthetic diesel like  Shell V power.

you dont need any

I only put a cup of Metholated spirits in my tank once a year to break down any water in the Tank

Should not need any.  The gasoline should be good as it comes.