If it is really backup, then propane.
Gas goes bad pretty quickly. Diesel also goes bad over a longer period but still goes bad.
Natural gas is dependent on the utilities to provide it. Debatable in a true emergency like an earthquake. Natural gas also has less energy per unit volume so generators generally produce less when compared to propane.
You can bury a large propane tank making it very robust, particularly in an emergency.
If the purpose is more than backup then diesel. Diesel engines are more reliable and last longer with less maintenance. If it is regularly used the diesel fuel will be consumed before it goes bad.
The generator has to be run every so often to keep it workinf properly and to recharge the battery that starts it. Don’t worry about the Diesel going bad you will be burning it off every so ofthen just startinf the generator to make sure it is working properly.
One thing to keep in mind is that propane is stored in liquid form, and requires heat to vaporize. Vaporization draws heat from the surrounding area and if the buried tank isn't big enough relative to the gas draw, the tank will 'freeze up' and you'll be stuck with no gas pressure.
I had to use a vaporizer (essentially a 'water heater' type unit) on my 10,000 gallon tank on the farm to alleviate the low gas pressure problem.