Bacterial spores are best destroyed by -

Autoclave: the principle is that water boils when its vapor pressure is equal to its surrounding pressure. Saturated steam has a penetrating power. When steam comes in contact with the cooler surface it condenses to water and gives up its latent heat. The large reduction in the volume sucks in more steam to the area and the process continues until the temperature of that area is raised to that of steam. The condensed water ensures moist condition for killing microbes present REF:Ananthanarayan & Panicker's Textbook of Microbiology 9th edition pg no:31