There isn’t necessarily one single type of fuel that is best, as they all have pretty major advantages and disadvantages. Gasoline is easily accessible, but also highly flammable, making it dangerous to keep large amounts on hand (it will also spoil if unused after a relatively short amount of time). Propane has a much longer shelf life, but propane tanks are also very large and obtrusive. Lastly, Diesel is the least flammable of the three, but will spoil after about two years if unused (still longer than gasoline).